19686: Introduce low-level api_client constructor

19686: Introduce low-level api_client constructor

When api() returns a ThreadSafeApiCache, we still want to provide a
mechanism to get a plain Resource object. The api_client() function is
that mechanism. It *just* builds the Resource object as api() did
before.

normalize_api_kwargs() takes the arguments passed to api() and turns
them into keyword arguments for api_client(). api_config_to_kwargs()
takes a configuration mapping nand turns them into keyword arguments for
api_client(). Both of these are small APIs, just returning one
dictionary from another.

With this reorganization, api(), api_from_config(), and
ThreadSafeApiClient() can all have simpler implementations.

Arvados-DCO-1.1-Signed-off-by: Brett Smith <brett.smith@curii.com>