BUILD: Add meson option for selective plugin building

[michal-shalev]

What?

Add a new build_plugins meson option to selectively build only specific plugins instead of all available plugins.

Why?

Building all plugins increases build time and requires all plugin dependencies to be present. When developing or deploying with only specific plugins (e.g., just UCX), building unnecessary plugins wastes time and resources. This option allows users to:

• Reduce build time significantly
• Avoid unnecessary dependencies
• Speed up development iteration when working on a single plugin

How?

• Added build_plugins meson option that accepts a comma-separated list of plugin names (e.g., UCX,GPUNETIO)
• Default value is empty string, which builds all available plugins (backward compatible)
• Created a global should_build_plugin dictionary that is computed once in the main meson.build and used throughout
• Applied plugin filtering to:
  • src/plugins/meson.build - only builds requested plugin libraries
  • test/unit/plugins/meson.build - only builds unit tests for enabled plugins
  • test/gtest/plugins/meson.build - skips if required plugins aren't enabled

Usage examples:

# Build only GDS plugin
meson setup build -Dbuild_plugins=GDS

# Build GDS and HF3FS plugins
meson setup build -Dbuild_plugins=GDS,HF3FS

# Build all plugins (default)
meson setup build

[github-actions]

👋 Hi michal-shalev! Thank you for contributing to ai-dynamo/nixl.

Your PR reviewers will review your contribution then trigger the CI to test your changes.

🚀

[iyastreb]

Seems to be useful
Maybe it can be just a regex implementing both allow and deny logic, similar to GTEST_FILTER?
E.g.
-Dbuild_plugins=GDS // allow, only GDS is used
-Dbuild_plugins=^GDS //deny, all except GDS are used

[ovidiusm]

Could we name this plugins?

[ovidiusm]

should_build_plugin should be called enabled_plugins IMO

[ovidiusm]

Let's make a list all_plugins and do case insensitive comparison to avoid duplicates

[aranadive]

@michal-shalev thanks for adding this. I think we can take a little bit better approach:
We should auto-generate a list of plugins during build by parsing the src/plugins directory for all the plugins. Then, create a map from plugin to plugin_directory. Then just iterate over this list in src/plugins/meson.build, ignoring ones in the user-provided mask. Just make sure all the dependencies for a plugin should be moved to the plugins meson.build and not handled in the src/plugins/meson.build.