Split out the `microgrid.client` to its own repository

[llucax]

What's needed?

All API clients are now being created in their own repositories. We need to do the same with the microgrid API, which now lives in the SDK.

Proposed solution

Split the frequenz.sdk.microgrid.client package to its own repository. But we also have a pending version update for the microgrid API and a new repository with helper function to write clients more easily.

• Create the new frequenz.client.microgrid repository and move the current SDK code there.
• Release a version using the same code as the SDK
• Remove the code from the SDK and depend on the new repo

Use cases

No response

Alternatives and workarounds

No response

Additional context

This will also help to improve the very complicated, multi-dimensional, problem we have involving dependency conflict of the SDK depending on the microgrid API v0.15 (and common v0.4) and the new API clients depending on common v0.5.