Add auth key support

[llucax]

What's needed?

We need to add an auth key to every request for authorization.

Proposed solution

Make the BaseApiClient take a key in the constructor and automatically pass the keys to stub methods.

Use cases

No response

Alternatives and workarounds

No response

Additional context

Example implementation:

• Add key based authorization to API Client frequenz-client-electricity-trading-python#29

But maybe we can use the AuthMetadataPlugin:

• https://grpc.io/docs/guides/auth/
• https://github.com/grpc/grpc/tree/master/examples/python/auth