Add a dispatch API client

[leandro-lucarella-frequenz]

What's needed?

We need to provide a dispatch API client.

Proposed solution

Write a client based on what we have for the microgrid API.

This should live in frequenz.sdk.dispatch.client to also follow the microgrid API.

Use cases

• Dispatch forwarding actor.
• Creation of dispatches by other actors.

Alternatives and workarounds

Use the dispatch API directly.

Additional context

Dispatching will be a core part of building actors and apps, so we need to include support for it in the SDK.

FYI @sahas-subramanian-frequenz @thomas-nicolai-frequenz

[leandro-lucarella-frequenz]

Blocked until we have the dispatch API open sourced.

[llucax]

This is done as a separate repo.