Common config for placers.

RunnerStatus is general information on Runner health as returned by Runner::Status() call.

Placer implements a placement strategy for calls that are load-balanced across runners in a pool.

Runner is the interface to invoke the execution of a function call on a specific runner.

RunnerCall provides access to the necessary details of request in order for it to be processed by a RunnerPool.

RunnerPool is the abstraction for getting an ordered list of runners to try for a call.