Adds a feature flag and the logic to hide plugin interactions on shared channels (#31185)

* Adds a feature flag and the logic to hide plugin interactions on shared channels

* Address review comments

* Fix CI

* Address review comments

* Address review comments

---------

Co-authored-by: Miguel de la Cruz <miguel@ctrlz.es>
Co-authored-by: Mattermost Build <build@mattermost.com>

server/public/model/feature_flags.go

@@ -22,6 +22,9 @@ type FeatureFlags struct {
 	// Enable DMs and GMs for shared channels.
 	EnableSharedChannelsDMs bool
 
+	// Enable plugins in shared channels.
+	EnableSharedChannelsPlugins bool
+
 	// AppsEnabled toggles the Apps framework functionalities both in server and client side
 	AppsEnabled bool
 
@@ -66,6 +69,7 @@ func (f *FeatureFlags) SetDefaults() {
 	f.TestBoolFeature = false
 	f.EnableRemoteClusterService = false
 	f.EnableSharedChannelsDMs = false
+	f.EnableSharedChannelsPlugins = true
 	f.AppsEnabled = false
 	f.NormalizeLdapDNs = false
 	f.DeprecateCloudFree = false