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/channels/store/sqlstore/remote_cluster_store.go

@@ -4,6 +4,7 @@
 package sqlstore
 
 import (
+	"database/sql"
 	"fmt"
 	"strings"
 
@@ -50,6 +51,19 @@ func (s sqlRemoteClusterStore) Save(remoteCluster *model.RemoteCluster) (*model.
 		return nil, err
 	}
 
+	// check for pluginID collisions - on collision treat as idempotent
+	if remoteCluster.PluginID != "" {
+		rc, err := s.GetByPluginID(remoteCluster.PluginID)
+		if err == nil {
+			// if this plugin id already exists, just return it
+			return rc, nil
+		}
+		if !errors.Is(err, sql.ErrNoRows) {
+			// anything other than NotFound is unexpected
+			return nil, errors.Wrapf(err, "failed to lookup RemoteCluster by pluginID %s", remoteCluster.PluginID)
+		}
+	}
+
 	query := `INSERT INTO RemoteClusters
 				(RemoteId, RemoteTeamId, Name, DisplayName, SiteURL, CreateAt,
 				LastPingAt, Token, RemoteToken, Topics, CreatorId, PluginID, Options)
@@ -69,6 +83,7 @@ func (s sqlRemoteClusterStore) Update(remoteCluster *model.RemoteCluster) (*mode
 		return nil, err
 	}
 
+	// not all fields can be updated.
 	query := `UPDATE RemoteClusters
 			SET Token = :Token,
 			RemoteTeamId = :RemoteTeamId,
@@ -129,6 +144,24 @@ func (s sqlRemoteClusterStore) Get(remoteId string) (*model.RemoteCluster, error
 	return &rc, nil
 }
 
+func (s sqlRemoteClusterStore) GetByPluginID(pluginID string) (*model.RemoteCluster, error) {
+	query := s.getQueryBuilder().
+		Select(remoteClusterFields("")...).
+		From("RemoteClusters").
+		Where(sq.Eq{"PluginID": pluginID})
+
+	queryString, args, err := query.ToSql()
+	if err != nil {
+		return nil, errors.Wrap(err, "remote_cluster_get_by_pluginid_tosql")
+	}
+
+	var rc model.RemoteCluster
+	if err := s.GetReplicaX().Get(&rc, queryString, args...); err != nil {
+		return nil, errors.Wrapf(err, "failed to find RemoteCluster by plugin_id")
+	}
+	return &rc, nil
+}
+
 func (s sqlRemoteClusterStore) GetAll(filter model.RemoteClusterQueryFilter) ([]*model.RemoteCluster, error) {
 	query := s.getQueryBuilder().
 		Select(remoteClusterFields("rc")...).