Add Unicast Channel

[leandro-lucarella-frequenz]

What's needed?

There is no unicast lightweight channel at the moment.

Proposed solution

Add a Unicast channel that can have only one sender and one receiver.

Use cases

No response

Alternatives and workarounds

Broadcast channel can be used instead, and just use one receiver.

Additional context

We did some tests in the past and the performance gains weren't as good as expected compared to BroadcastChannel. It needs more thought (and maybe compelling use cases to dig further).