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/post.go

@@ -96,6 +96,7 @@ type Post struct {
 	FileIds       StringArray     `json:"file_ids,omitempty"`
 	PendingPostId string          `json:"pending_post_id" db:"-"`
 	HasReactions  bool            `json:"has_reactions,omitempty"`
+	RemoteId      *string         `json:"remote_id,omitempty"`
 
 	// Transient data populated before sending a post to the client
 	ReplyCount   int64         `json:"reply_count" db:"-"`
@@ -206,6 +207,7 @@ func (o *Post) ShallowCopy(dst *Post) error {
 	dst.Participants = o.Participants
 	dst.LastReplyAt = o.LastReplyAt
 	dst.Metadata = o.Metadata
+	dst.RemoteId = o.RemoteId
 	return nil
 }
 
@@ -235,6 +237,18 @@ type GetPostsSinceOptions struct {
 	SkipFetchThreads         bool
 	CollapsedThreads         bool
 	CollapsedThreadsExtended bool
+	SortAscending            bool
+}
+
+type GetPostsSinceForSyncOptions struct {
+	ChannelId       string
+	Since           int64 // inclusive
+	Until           int64 // inclusive
+	SortDescending  bool
+	ExcludeRemoteId string
+	IncludeDeleted  bool
+	Limit           int
+	Offset          int
 }
 
 type GetPostsOptions struct {
@@ -452,6 +466,11 @@ func (o *Post) IsSystemMessage() bool {
 	return len(o.Type) >= len(POST_SYSTEM_MESSAGE_PREFIX) && o.Type[:len(POST_SYSTEM_MESSAGE_PREFIX)] == POST_SYSTEM_MESSAGE_PREFIX
 }
 
+// IsRemote returns true if the post originated on a remote cluster.
+func (o *Post) IsRemote() bool {
+	return o.RemoteId != nil && *o.RemoteId != ""
+}
+
 func (o *Post) IsJoinLeaveMessage() bool {
 	return o.Type == POST_JOIN_LEAVE ||
 		o.Type == POST_ADD_REMOVE ||