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/app/remote_cluster.go

@@ -4,6 +4,7 @@
 package app
 
 import (
+	"database/sql"
 	"net/http"
 
 	"github.com/pkg/errors"
@@ -14,6 +15,57 @@ import (
 	"github.com/mattermost/mattermost/server/public/model"
 )
 
+func (a *App) RegisterPluginForSharedChannels(opts model.RegisterPluginOpts) (remoteID string, err error) {
+	// check for pluginID already registered
+	rc, err := a.Srv().Store().RemoteCluster().GetByPluginID(opts.PluginID)
+	if err != nil {
+		if !errors.Is(err, sql.ErrNoRows) {
+			// anything other than not_found is unrecoverable
+			return "", err
+		}
+	}
+
+	// if plugin is already registered then treat this as an update.
+	if rc != nil {
+		rc.DisplayName = opts.Displayname
+		rc.Options = opts.GetOptionFlags()
+
+		if _, err = a.Srv().Store().RemoteCluster().Update(rc); err != nil {
+			return "", err
+		}
+		return rc.RemoteId, nil
+	}
+
+	rc = &model.RemoteCluster{
+		Name:        opts.Displayname,
+		DisplayName: opts.Displayname,
+		Token:       model.NewId(),
+		CreatorId:   opts.CreatorID,
+		PluginID:    opts.PluginID,
+		Options:     opts.GetOptionFlags(),
+	}
+
+	rcSaved, err := a.Srv().Store().RemoteCluster().Save(rc)
+	if err != nil {
+		return "", err
+	}
+
+	return rcSaved.RemoteId, nil
+}
+
+func (a *App) UnregisterPluginForSharedChannels(pluginID string) error {
+	rc, err := a.Srv().Store().RemoteCluster().GetByPluginID(pluginID)
+	if err != nil {
+		return err
+	}
+
+	_, appErr := a.DeleteRemoteCluster(rc.RemoteId)
+	if appErr != nil {
+		return appErr
+	}
+	return nil
+}
+
 func (a *App) AddRemoteCluster(rc *model.RemoteCluster) (*model.RemoteCluster, *model.AppError) {
 	rc, err := a.Srv().Store().RemoteCluster().Save(rc)
 	if err != nil {