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/upload_session.go

@@ -42,6 +42,10 @@ type UploadSession struct {
 	// The amount of received data in bytes. If equal to FileSize it means the
 	// upload has finished.
 	FileOffset int64 `json:"file_offset"`
+	// Id of remote cluster if uploading for shared channel
+	RemoteId string `json:"remote_id"`
+	// Requested file id if uploading for shared channel
+	ReqFileId string `json:"req_file_id"`
 }
 
 // ToJson serializes the UploadSession into JSON and returns it as string.