Shared channels plugin APIs for MS Teams plugin (#25805)

New plugin APIs and hooks for accessing Shared Channels service via plugin. 

- RegisterPluginForSharedChannels(opts model.RegisterPluginOpts) (remoteID string, err error)
- UnregisterPluginForSharedChannels(pluginID string) error
- ShareChannel(sc *model.SharedChannel) (*model.SharedChannel, error)
- UpdateSharedChannel(sc *model.SharedChannel) (*model.SharedChannel, error)
- UnshareChannel(channelID string) (unshared bool, err error)
- UpdateSharedChannelCursor(channelID, remoteID string, cusror model.GetPostsSinceForSyncCursor) error
- SyncSharedChannel(channelID string) error
- InviteRemoteToChannel(channelID string, remoteID string, userID string) error
- UninviteRemoteFromChannel(channelID string, remoteID string) error

Hooks
- OnSharedChannelsSyncMsg(msg *model.SyncMsg, rc *model.RemoteCluster) (model.SyncResponse, error)
- OnSharedChannelsPing(rc *model.RemoteCluster) bool

server/public/model/shared_channel.go

@@ -297,3 +297,21 @@ type SyncResponse struct {
 	ReactionsLastUpdateAt int64    `json:"reactions_last_update_at"`
 	ReactionErrors        []string `json:"reaction_errors"`
 }
+
+// RegisterPluginOpts is passed by plugins to the `RegisterPluginForSharedChannels` plugin API
+// to provide options for registering as a shared channels remote.
+type RegisterPluginOpts struct {
+	Displayname  string // a displayname used in status reports
+	PluginID     string // id of this plugin registering
+	CreatorID    string // id of the user/bot registering
+	AutoShareDMs bool   // when true, all DMs are automatically shared to this remote
+}
+
+// GetOptionFlags returns a Bitmask of option flags as specified by the boolean options.
+func (po RegisterPluginOpts) GetOptionFlags() Bitmask {
+	var flags Bitmask
+	if po.AutoShareDMs {
+		flags |= BitflagOptionAutoShareDMs
+	}
+	return flags
+}