MM-16809: Added EnsureChannel Helper (#11852)

This PR adds EnsureChannel helper analogous to EnsureBot helper which creates a new channel if not exists and update the meta data of the channel if exists. Also, it will throw error
if the type of existing channel do not match the type of new channel.

plugin/client.go

@@ -10,6 +10,7 @@ import (
 const (
 	INTERNAL_KEY_PREFIX = "mmi_"
 	BOT_USER_KEY        = INTERNAL_KEY_PREFIX + "botid"
+	CHANNEL_KEY         = INTERNAL_KEY_PREFIX + "channelid"
 )
 
 // Starts the serving of a Mattermost plugin over net/rpc. gRPC is not yet supported.