Update Chat::Responder module

What does this MR do and why?

Why?

In order to unblock/continue with !109197 (merged) we are introducing the following changes.

What?

As per Update Chat::Responder (#388796 - closed), #388796 (comment 1252940727) and #388796 (comment 1249916796), we are selecting a chat responder based on the response_url value.

If the response_url starts with https://hooks.slack.com/, it will respond to Gitlab::Chat::Responder::Slack, and in any other case to Gitlab::Chat::Responder::Mattermost.

The functionality will be available behind the disable_chat_name_integration feature flag.

How to set up and validate locally

N/A

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #388796 (closed)