Add buf plugin push command

[emcfarlane]

This adds the buf plugin push command to upload Buf check plugins to the BSR. Plugins must implement the PluginRPC framework. See the Buf plugins overview for details. The Plugin must be a WebAssembly binary, that implements the check service.

The output for buf plugin push --help:

Push a check plugin to a registry

The first argument is the plugin full name in the format <remote/owner/plugin>.

Usage:
  buf plugin push <remote/owner/plugin> [flags]

Flags:
      --binary string               The path to the Wasm binary file to push.
      --create                      Create the plugin if it does not exist. Defaults to creating a private plugin on the BSR if --create-visibility is not set. Must be used with --create-type.
      --create-type string          The plugin's type setting, if created. Can only be set with --create-type. Must be one of [check]
      --create-visibility string    The module's visibility setting, if created. Can only be set with --create. Must be one of [public,private] (default "private")
  -h, --help                        help for push
      --label strings               Associate the label with the plugins pushed. Can be used multiple times.
      --source-control-url string   The URL for viewing the source code of the pushed plugins (e.g. the specific commit in source control).

[github-actions]

The latest Buf updates on your PR. Results from workflow Buf CI / buf (pull_request).

Build	Format	Lint	Breaking	Updated (UTC)
✅ passed	✅ passed	✅ passed	✅ passed	Dec 3, 2024, 9:05 PM

[doriable]

Chatted offline, some of this should live with bufpluginapi, similar to Upload for modules :)

[doriable]

Looks good to me!