[Platform] SavedObject Schemas

[rudolf]

Allow plugins to configure the values in the SavedObjectsSchemaTypeDefinition for each of their Saved Object types.

interface SavedObjectsSchemaTypeDefinition {
  isNamespaceAgnostic: boolean;
  hidden?: boolean;
  indexPattern?: (() => string) | string; // Note: remove injected config: Config param
  convertToAliasScript?: string; // rename to reindexScript?
}

Blocked by #52071

[elasticmachine]

Pinging @elastic/kibana-platform (Team:Platform)