Support "Socket Mode" for Slack channel

[justinfiore]

In OpenClaw, I use the "Socket Mode" version of the Slack channel so that I don't have to open inbound traffic to my instance for Slack webhooks.
In IronClaw, it appears that at present, the built-in Slack channel only supports normal incoming webhooks to receive messages.

Here are the docs for Socket Mode: https://docs.slack.dev/apis/events-api/using-socket-mode/

This issue is to add support for Socket Mode for the Slack channel for even more privacy and security.

Here is how the "Socket Mode" Configuration in the "Slack App" configuration screen looks:

[KonstantinMirin]

You must be kidding: #333
I did this a month ago, addressed all issues, still no merge and now -- issue to develop it :-)

[justinfiore]

@KonstantinMirin I created this issue because I wanted it to exist (I'm not a core developer or anything. just a guy like you).
Would you be willing to rebase it onto the latest staging and get your PR reopened or create a new PR?
I have the start of a branch, but I'm still working on testing it out and it sounds like you are much further along.