Add jamulusserver/privateChatMessage RPC method for sending messages to single channels

[dingodoppelt]

Short description of changes

This makes it possible to query the server over RPC to send a chat text to a connected channel. The method takes the chat text and the channel id as arguments.

CHANGELOG: Add jamulusserver/privateChatMessage RPC method for sending messages to single channels

Context: Fixes an issue?
No

Does this change need documentation? What needs to be documented and how?

Part of auto generated RPC docs

Status of this Pull Request

Requesting review

Checklist

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[pljones]

Thanks.