Internal: add indices setter to IndicesRequest interface

We currently expose generic getters for `indices` and `indicesOptions` on the `IndicesRequest` interface. This commit adds a generic setter as well, which can be used to set the indices to a request. The setter impl throws `UnsupportedOperationException` if called on internal requests, since it makes sense to eventually replace the indices on external requests only. Also throws exception when the set operation doesn't make sense.