Allow users to set the scheduled executor service

[mziccard]

grpc-based services use a ScheduledExecutorService. We should allow users to set an executor other than the default one in the service options.

Related to this: should we create an abstract option class for grpc-based services (that extends ServiceOptions?) That could expose some of the grpc-only configurations like setting the executor or converting the connection timeouts (see here).

/cc @aozarov @ajkannan

[ajkannan]

+1 for creating an abstract options subclass of ServiceOptions for gRPC configs

[aozarov]

I have the feeling that we may need to have one top-level abstract service options and 2 other abstract sub classes for apiclient and gRPC. Aside from executor there are other differences such as the way timeout is handled.

[mziccard]

This was closed in #1011