MM-27493 Shared channels (MVP) (#17301)

Remote Cluster Service
- provides ability for multiple Mattermost cluster instances to create a trusted connection with each other and exchange messages
- trusted connections are managed via slash commands (for now)
- facilitates features requiring inter-cluster communication, such as Shared Channels
Shared Channels Service
- provides ability to shared channels between one or more Mattermost cluster instances (using trusted connection)
- sharing/unsharing of channels is managed via slash commands (for now)

model/session.go

@@ -26,6 +26,7 @@ const (
 	SESSION_PROP_IS_BOT_VALUE         = "true"
 	SESSION_TYPE_USER_ACCESS_TOKEN    = "UserAccessToken"
 	SESSION_TYPE_CLOUD_KEY            = "CloudKey"
+	SESSION_TYPE_REMOTECLUSTER_TOKEN  = "RemoteClusterToken"
 	SESSION_PROP_IS_GUEST             = "is_guest"
 	SESSION_ACTIVITY_TIMEOUT          = 1000 * 60 * 5 // 5 minutes
 	SESSION_USER_ACCESS_TOKEN_EXPIRY  = 100 * 365     // 100 years