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 +1,47 @@
+# 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 fetch
+
+# download the latest 1.19 envtest, and print out the path
+setup-envtest fetch -k '1.19.+' -p path
+
+# switch to the most recent 1.21 envtest on disk
+source <(setup-envtest switch -k '1.21.*' -p env)
+
+# list all available local versions for darwin/amd64
+setup-envtest list-installed --os darwin --arch amd64
+
+# remove all 1.16 versions from disk
+setup-envtest cleanup -k '1.16.*'
+```
+
+## 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.