Support UID for playlists

[julienduchesne]

This is how Grafana 9.0 manages playlists now
This is done in a way that's retrocompatible with Grafana 8 though (id or UID accepted in functions)

[inkel]

Changes LGTM, but given that the changes for *Client.Playlist, *Client.NewPlaylist, and *Client.UpdatePlaylist are all breaking changes (we're changing a public API), we probably should increment the major version when releasing this.

[julienduchesne]

Changes LGTM, but given that the changes for *Client.Playlist, *Client.NewPlaylist, and *Client.UpdatePlaylist are all breaking changes (we're changing a public API), we probably should increment the major version when releasing this.

Major is zero. According to semver, I can break the API on a minor 😄

[inkel]

Hah! Good point.