[MM-62066] Allow using plugin settings defined in manifest as fallback when using custom sections and plugin is disabled (#29500)

* Allow using plugin settings defined in manifest as fallback when using custom sections and plugin is disabled

* Update server/public/model/manifest.go

Co-authored-by: Christopher Poile <cpoile@gmail.com>

* Add custom setting case

---------

Co-authored-by: Christopher Poile <cpoile@gmail.com>
Co-authored-by: Mattermost Build <build@mattermost.com>

webapp/platform/types/src/plugins.ts

@@ -55,6 +55,7 @@ export type PluginSettingSection = {
     header?: string;
     footer?: string;
     custom?: boolean;
+    fallback?: boolean;
 };
 
 export type PluginSetting = {