Add gRPC client testing utilities

This commit introduces a new module which provides a framework for
testing gRPC client implementations.

The module simplifies mocking gRPC stub interactions and asserting
client behavior for both unary and streaming calls. It allows defining
test cases in separate Python files with a conventional structure,
making tests more organized and maintainable.

Key features include:
- Discovery of test case files based on directory structure.
- Mocking of gRPC responses and errors.
- Assertion helpers for gRPC requests and client results/exceptions.
- Support for unary-unary and unary-stream gRPC methods.

Detailed usage instructions and examples are available in the module's
docstring.

Please note this module is added here temporarily and will be moved to
the `frequenz-client-base` repository as a general test utility in the
near future.

Signed-off-by: Leandro Lucarella <[email protected]>

Author: llucax