Added EMIT_PEER protocol

[joente]

Description

In some cases, you want to communicate by a room but only want to emit to peers, not echo back to yourself.

This pull request introduces a separate protocol, EMIT_PEER which does exactly that.

Clients can implement this protocol, for example as room.emit(..., peers_only: bool = False).

Type of change

• New feature (non-breaking change which adds functionality)

How Has This Been Tested?

• Test Room

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• New and existing unit tests pass locally with my changes