Envtest Binary Version Manager Tool

This introduces an envtest binary manager tool.
It can download binaries from GCS, list available versions,
print out help for switching between them, and remove old ones.

By default, it downloads binaries into an OS-specific cache directory,
as per the OS's conventions, but this can be overridden.

tools/setup-envtest/README.md

@@ -0,0 +1,108 @@
+# Envtest Binaries Manager
+
+This is a small tool that manages binaries for envtest. It can be used to
+download new binaries, list currently installed and available ones, and
+clean up versions.
+
+To use it, just go-install it on 1.16+ (it's a separate, self-contained
+module):
+
+```shell
+go install sigs.k8s.io/controller-runtime/tools/setup-envtest@latest
+```
+
+For full documentation, run it with the `--help` flag, but here are some
+examples:
+
+```shell
+# download the latest envtest, and print out info about it
+setup-envtest use
+
+# download the latest 1.19 envtest, and print out the path
+setup-envtest use -k '1.19.x!' -p path
+
+# switch to the most recent 1.21 envtest on disk
+source <(setup-envtest use -i -k '1.21.x' -p env)
+
+# list all available local versions for darwin/amd64
+setup-envtest list -i --os darwin --arch amd64
+
+# remove all 1.16 versions from disk
+setup-envtest cleanup -k '1.16.x'
+
+# use the value from $KUBEBUILDER_ASSETS if set, otherwise follow the normal
+# logic for 'use'
+setup-envtest --use-env=always
+
+# use the value from $KUBEBUILDER_ASSETS if set, otherwise use the latest
+# installed version
+setup-envtest use -i --use-env=always
+
+# sideload a pre-downloaded tarball as Kubernetes 1.16.2 into our store
+setup-envtest sideload -k '1.16.2' < downloaded-envtest.tar.gz
+```
+
+## Where does it put all those binaries?
+
+By default, binaries are stored in an OS-specific cache directory, as per
+the OS's conventions (see Go's `os.UserCacheDir`).
+
+For example, on Linux this is `$XDG_CACHE_DIR`, or just `~/.config` if
+that's unset.
+
+There's an overall folder that holds all files, and inside that is
+a folder for each version/platform pair.  The exact directory structure is
+not guarnateed, except that the leaf directory will contain the names
+expected by envtest.  You should always use `setup-envtest fetch` or
+`setup-envtest switch` (generally with the `-p path` or `-p env` flags) to
+get the directory that you should use.
+
+## What if I don't want to talk to the internet?
+
+There are a few options.
+
+First, you'll probably want to set the `-i/--installed` flag. If you want
+to avoid forgetting to set this flag, set  the `ENVTEST_INSTALLED_ONLY`
+env variable, which will switch that flag on by default.
+
+Then, you have a few options for managing your binaries:
+
+- If you don't *really* want to manage with this tool, or you want to
+  respect the $KUBEBUILDER_ASSETS variable if it's set to something
+  outside the store, use the `use --use-env=always -i` command.
+
+  `--use-env=always` makes the command unconditionally use the value of
+  KUBEBUILDER_ASSETS as long as it contains the required binaries, and
+  `-i` indicates that we only ever want to work with installed binaries
+  (no reaching out the the remote GCS storage).
+
+  As noted about, you can use `ENVTEST_INSTALLED_ONLY=true` to switch `-i`
+  on by default, and you can use `ENVTEST_USE_ENV=always` to switch
+  `--use-env=always` on by default.
+
+- If you want to use this tool, but download your gziped tarballs
+  separately, you can use the `sideload` command.  You'll need to use the
+  `-k/--version` flag to indicate which version you're sideloading.
+
+  After that, it'll be as if you'd installed the binaries with `use`.
+
+- If you want to talk to some internal source, you can use the
+  `--remote-bucket` and `--remote-server` options.  The former sets which
+  GCS bucket to download from, and the latter sets the host to talk to as
+  if it were a GCS endpoint. Theoretically, you could use the latter
+  version to run an internal "mirror" -- the tool expects
+
+  - `HOST/storage/v1/b/BUCKET/o` to return JSON like
+
+    ```json
+    {"items": [
+        {"name": "kubebuilder-tools-X.Y.Z-os-arch.tar.gz", "md5Hash": "<base-64-encoded-md5-hash>"},
+        {"name": "kubebuilder-tools-X.Y.Z-os-arch.tar.gz", "md5Hash": "<base-64-encoded-md5-hash>"},
+    ]}
+    ```
+
+  - `HOST/storage/v1/b/BUCKET/o/TARBALL_NAME` to return JSON like
+    `{"name": "kubebuilder-tools-X.Y.Z-os-arch.tar.gz", "md5Hash": "<base-64-encoded-md5-hash>"}`
+
+  - `HOST/storage/v1/b/BUCKET/o/TARBALL_NAME?alt=media` to return the
+    actual file contents