Add sender support for chat channels (recipient field mapping)

Problem/Motivation

Core notifier project already has mapping for sms and email

For chat like services, it may make sense to have some kind of equivalent.

E.g a field mapping to Slack username or channel on an entity.

That way, the Notifier Sender service can be used, and can figure out destinations, without needing to call Notification::channels() manually.

This might mean a whole unique field mapping setup for each service.

Some may need specific logic, like Slack defaults to channel unless prefixed with '@' in which case a user is used. We should be able to either require a # / @ prefix in field values and we transform it when creating a recipient. And/or provide extra parameter config, suchthat a non-prefixed field value is transformed to a channel (#) or user (@).

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes