Plugin API for Shared Channels: support auto invite (#25834)

* option for auto inviting plugin to all shared channels.

* auto-invite remotes to shared channels when flag set

* fix unit test

---------

Co-authored-by: Mattermost Build <build@mattermost.com>

server/public/model/shared_channel.go

@@ -305,6 +305,7 @@ type RegisterPluginOpts struct {
 	PluginID     string // id of this plugin registering
 	CreatorID    string // id of the user/bot registering
 	AutoShareDMs bool   // when true, all DMs are automatically shared to this remote
+	AutoInvited  bool   // when true, the plugin is automatically invited and sync'd with all shared channels.
 }
 
 // GetOptionFlags returns a Bitmask of option flags as specified by the boolean options.
@@ -313,5 +314,8 @@ func (po RegisterPluginOpts) GetOptionFlags() Bitmask {
 	if po.AutoShareDMs {
 		flags |= BitflagOptionAutoShareDMs
 	}
+	if po.AutoInvited {
+		flags |= BitflagOptionAutoInvited
+	}
 	return flags
 }