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/channel.go

@@ -142,6 +142,14 @@ type ChannelMemberCountByGroup struct {
 	ChannelMemberTimezonesCount int64  `db:"-" json:"channel_member_timezones_count"`
 }
 
+type ChannelOption func(channel *Channel)
+
+func WithID(ID string) ChannelOption {
+	return func(channel *Channel) {
+		channel.Id = ID
+	}
+}
+
 func (o *Channel) DeepCopy() *Channel {
 	copy := *o
 	if copy.SchemeId != nil {