Add plugin management capabilities to horizon

Horizon plugins are python files that must be placed in specific
location.

This change is adding facilities to enable easy plugin management from
the charm.

Adding the script `plugin_management.py`, with the following usage:
```
Usage: plugin_management.py <command> <plugin>...
Commands: enable, disable
```

Example:
```
/usr/bin/plugin_management.py enable magnum
Enabled '_1370_project_container_infra_panel_group.py'
Enabled '_1371_project_container_infra_clusters_panel.py'
Enabled '_1372_project_container_infra_cluster_templates_panel.py'
Enabled '_2370_admin_container_infra_panel_group.py'
Enabled '_2371_admin_container_infra_quotas_panel.py'
```

The plugin will lookup the file
`/usr/lib/python3/dist-packages/openstack_dashboard/available/<plugin
name>`. If this file is missing, aborts.

This file should contain the name of the files necessary to enable the
Horizon plugin. (only basenames)

rocks/horizon/python/plugin_management.py

@@ -0,0 +1,86 @@
+#!/usr/bin/env python3
+# Copyright 2023 Canonical Ltd.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+import sys
+from pathlib import Path
+
+OS_DASHBOARD = Path("/usr/lib/python3/dist-packages/openstack_dashboard/")
+ENABLED = OS_DASHBOARD / "enabled"
+AVAILABLE = OS_DASHBOARD / "available"
+
+
+def _get_plugin_files(plugin: str) -> list[str]:
+    files = AVAILABLE / plugin
+    if not files.exists():
+        raise Exception(
+            f"No plugin files at {str(files)!r}, is {plugin!r} a supported plugin?"
+        )
+    return files.read_text().splitlines()
+
+
+def enable_plugin(plugin: str):
+    files = _get_plugin_files(plugin)
+    for file in files:
+        enabled = ENABLED / file
+        available = AVAILABLE / file
+        if enabled.exists():
+            continue
+        if not available.exists():
+            raise Exception(f"Plugin file {str(available)!r} does not exist")
+        enabled.symlink_to(available)
+        print(f"Enabled {file!r}")
+
+
+def disable_plugin(plugin: str):
+    files = _get_plugin_files(plugin)
+    for file in files:
+        enabled = ENABLED / file
+        if not enabled.exists():
+            continue
+        enabled.unlink()
+        print(f"Disabled {file!r}")
+
+
+COMMANDS = {
+    "enable": enable_plugin,
+    "disable": disable_plugin,
+}
+
+
+def main(cmd: str, plugins: list[str]):
+    command = COMMANDS.get(cmd)
+    if not command:
+        print(f"Unknown command {cmd!r}")
+        sys.exit(1)
+
+    for plugin in plugins:
+        command(plugin)
+
+
+def _usage():
+    print("Usage: plugin_management.py <command> <plugin>...")
+    print("Commands: " + ", ".join(COMMANDS.keys()))
+
+
+if __name__ == "__main__":
+    args = sys.argv[1:]
+    if "-h" in args or "--help" in args:
+        _usage()
+    else:
+        if len(args) < 2:
+            _usage()
+            sys.exit(1)
+        cmd, *plugins = args
+        main(cmd, plugins)

rocks/horizon/rockcraft.yaml

@@ -45,3 +45,31 @@ parts:
     override-prime: |
       craftctl default
       echo > $CRAFT_PRIME/etc/apache2/ports.conf
+    
+  horizon-plugin:
+    plugin: dump
+    source: python/
+    organize:
+     plugin_management.py: usr/bin/plugin_management.py
+    override-prime: |
+      craftctl default
+      chmod 550 $CRAFT_PRIME/usr/bin/plugin_management.py
+
+  horizon-magnum-plugin:
+    after: [horizon]
+    plugin: nil
+    stage-packages:
+      - python3-magnum-ui
+    organize:
+      usr/lib/python3/dist-packages/openstack_dashboard/enabled/_1370_project_container_infra_panel_group.py: usr/lib/python3/dist-packages/openstack_dashboard/available/_1370_project_container_infra_panel_group.py
+      usr/lib/python3/dist-packages/openstack_dashboard/enabled/_1371_project_container_infra_clusters_panel.py: usr/lib/python3/dist-packages/openstack_dashboard/available/_1371_project_container_infra_clusters_panel.py
+      usr/lib/python3/dist-packages/openstack_dashboard/enabled/_1372_project_container_infra_cluster_templates_panel.py: usr/lib/python3/dist-packages/openstack_dashboard/available/_1372_project_container_infra_cluster_templates_panel.py
+      usr/lib/python3/dist-packages/openstack_dashboard/enabled/_2370_admin_container_infra_panel_group.py: usr/lib/python3/dist-packages/openstack_dashboard/available/_2370_admin_container_infra_panel_group.py
+      usr/lib/python3/dist-packages/openstack_dashboard/enabled/_2371_admin_container_infra_quotas_panel.py: usr/lib/python3/dist-packages/openstack_dashboard/available/_2371_admin_container_infra_quotas_panel.py
+    override-prime: |
+      craftctl default
+      echo "_1370_project_container_infra_panel_group.py
+      _1371_project_container_infra_clusters_panel.py
+      _1372_project_container_infra_cluster_templates_panel.py
+      _2370_admin_container_infra_panel_group.py
+      _2371_admin_container_infra_quotas_panel.py" > $CRAFT_PRIME/usr/lib/python3/dist-packages/openstack_dashboard/available/magnum