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)

services/remotecluster/error.go

@@ -0,0 +1,24 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
+package remotecluster
+
+import "fmt"
+
+type BufferFullError struct {
+	capacity int
+}
+
+func NewBufferFullError(capacity int) BufferFullError {
+	return BufferFullError{
+		capacity: capacity,
+	}
+}
+
+func (e BufferFullError) Capacity() int {
+	return e.capacity
+}
+
+func (e BufferFullError) Error() string {
+	return fmt.Sprintf("buffer capacity (%d) exceeded", e.capacity)
+}