Add a recipient interface channel+/transport channelrouter

In the case where multiple chat channels are configured, greedy channels like Mercure (supports() has minimum requirements), ensure filters filter out for example where a message needs to go to Slack and never mercure.

To prevent notifications getting routed to catch-all channels, like Chat, this should by default error out (exception) if a notification type is not covered by a configured interface -> channel/transport mapping

This should help with scenarios like:

Notification is routing to a Slack recipient.
The mercure transport is enabled (is a chat channel transport)
The mercure transport will accept the notification with slack recipient by default.

We add something that can limit unintended transports.

\Drupal\notifier_chat_channel\Transport\Slack\Recipient\SlackRecipientInterface: chat/slack

IF a message is routed which dosent match a class/iface/abstract etc combo, then throw exception.

This should reduce the number of custom channelrouter services needed, and also help to prevent unintended message routing if for example more transports within a channel are added progressively.