[FEATURE] Make NamedRoute a Writeable

[DarshitChanpura]

Is your feature request related to a problem?

In opensearch-project/OpenSearch#7957, ActionModule modifies a declaration of registerDynamicRoute to support the newly added NamedRoutes. But current implementation in SDK passes these values as String to then be parsed inside registerDynamicRoute. There is a better way to handle this scenario.

What solution would you like?

Per @dbwiddis 's comment, NamedRoute can be turned into a Writeable to allow parsing it to an Object in a much cleaner way.