Plugins: Allow explicitly specifying the builtin version of a plugin

[tomhjp]

For example, if you have registered a versioned kubernetes auth plugin, omitting the version when mounting kubernetes auth will always select the versioned plugin, but you can specify the builtin version instead like so:

vault auth enable -plugin-version="v0.14.0+builtin" kubernetes

Also disallows registering any plugins with a builtin identifier in the metadata. I referenced here for the format and terminology of the tag metadata.

[swenson]

This looks good in general, but when I tested it locally, I don't seem to get the version I requested:

$ vault plugin register -version=v1.2.4 -sha256=$(sha256sum dev-plugins/kv | cut -d' ' -f1) secret kv
$ vault secrets enable -plugin-version=v1.2.4 -path=kv124 kv
$ curl http://127.0.0.1:8200/v1/sys/mounts
...
    "kv124/": {
      "accessor": "kv_a2e94c5b",
      "config": {
        "default_lease_ttl": 0,
        "force_no_cache": false,
        "max_lease_ttl": 0
      },
      "deprecation_status": "supported",
      "description": "",
      "external_entropy_access": false,
      "local": false,
      "options": null,
      "plugin_version": "v1.2.4",
      "running_plugin_version": "v0.13.0+builtin",
      "running_sha256": "",
      "seal_wrap": false,
      "type": "kv",
      "uuid": "0a11311f-2b26-1294-87d0-0c34c6c218ed"
    },
    ...

Looks like the plugin_version is correctly requested, but it's still running the builtin version?

[swenson]

LGTM

The kv plugin seems to be special and always selects the builtin, but other plugin types seem fine.