[CLIENT] Remove unnecessary intermediate interfaces

`Client`, `ClusterAdminClient` and `IndicesAdminClient` had corresponding intermediate `internal` interfaces that are unnecessary and cause a lot of casting. This commit removes the intermediate interfaces and uses the super interfaces directly.

This PR also  simplifys `FilterClient` by subclassing `AbstractClient`
I think it can be named `IndicesAdminClient` (the fqn already suggests it's filtered `FilterClient.IndicesAdminClient`) or even `IndicesAdmin` (the fqn also suggests it's a client) - `FilterClient.IndicesAdmin`. Or... `FilterAdminClient.Indices` (<- I like that one the most) ?