Update BaseRestHandler to implement default routes

[saratvemulapalli]

Describe the bug
Update BaseRestHandler to implement default routes() instead of an abstract class.
This will help developers not to implement empty methods even though they don't need it.
They can always override the method if they'd like.

OpenSearch/server/src/main/java/org/opensearch/rest/BaseRestHandler.java

Line 102 in 0ba0e7c

public abstract List<Route> routes();

Expected behavior
Return an empty list so that classes which extend do not have implement empty methods even if they don't have to.

Plugins
OpenSearch Anomaly-Detection

Host/Environment (please complete the following information):

• OS: Linux x64

[dblock]

In this case routes is not required because all the routes have replacedRoutes.

[VijayanB]

routes() method is from interface RestHandler, which already has default implementation here. if we remove this method from BaseRestHandler, we don't have to override at subclass extending BaseRestHandler.

[Bukhtawar]

Can we have a way to enforce one of routes or replacedRoute as I don't think a RestHandler should exist without a route. This proposal can allow the plugin implementation to get away without providing any of them which breaks the contract

[vrozov]

Such a contract can't be enforced by Java. Any subclass that implements RestHandler can overwrite both routes() and replacedRoutes() and return empty list for both. Additionally, I don't think that enforcing that one of the methods returns a non-empty list is desirable. Some handlers may provide routes only under certain conditions/settings.