Ability to override OperationsClient configuration.

[lukesneeringer]

Currently, there is a bug in GAX/GAPICs that makes it impossible to specify rpc timeout configuration for OperationsClient.

Essentially, any user overrides are passed to the operations client, but the per-API defaults are not (see here in Speech).

We should move to a system where the config and override are merged into one common configuration much earlier, and read from that, and send it to operations client as an override.