This PR adds a base client and implements some of the surface.  All the
streaming client abstractions are elided and will be introduced in
subsequent PRs, but this PR does include non-streaming RPC methods

Alongside the client, we introduce an option type (WriterOption) for
constructing a client in a variadic fashion.

The client contains an internal settings type, streamSettings, which
contains fields of note for both the streaming client abstraction and
its flow controller.

Testing: This PR contains unit tests, but doesn't include integration
tests.  I'll start hoisting that in soon.