Implements:

• Download and install missing plugins #3

• Added new command fabric install which install plugins into {{cacheDir}}/plugins and creates .fabric-lock.json file containing locks for plugins on all platforms.

  Use --upgrade or -u flag with fabric install to download missing or updated plugins in version constraint list under global configuration.

• Global config has new property fabric.plugin_registry.base_url (default: https://registry.blackstork.io)

   fabric {
      plugin_registry {
         base_url = "https://registry.blackstork.io"
      }
   }

• Removed --plugin-dir flag because it could cause issues. It's best to use global config parameter fabric.plugin_registrty.mirror_dir instead since it ensures this property stays the same when running different install, render, data commands in sequence.

• These changes also include updated .goreleaser.yaml and tools/pluginmeta script to bundle each plugin into separate archive and make release parsable for Fabric Registry.