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)

app/authentication.go

@@ -20,6 +20,7 @@ const (
 	TokenLocationCookie
 	TokenLocationQueryString
 	TokenLocationCloudHeader
+	TokenLocationRemoteClusterHeader
 )
 
 func (tl TokenLocation) String() string {
@@ -34,6 +35,8 @@ func (tl TokenLocation) String() string {
 		return "QueryString"
 	case TokenLocationCloudHeader:
 		return "CloudHeader"
+	case TokenLocationRemoteClusterHeader:
+		return "RemoteClusterHeader"
 	default:
 		return "Unknown"
 	}
@@ -291,5 +294,9 @@ func ParseAuthTokenFromRequest(r *http.Request) (string, TokenLocation) {
 		return token, TokenLocationCloudHeader
 	}
 
+	if token := r.Header.Get(model.HEADER_REMOTECLUSTER_TOKEN); token != "" {
+		return token, TokenLocationRemoteClusterHeader
+	}
+
 	return "", TokenLocationNotFound
 }