Add support for a simple Slack connection

[instantiator]

Context

It's time to add a method for posting to Slack.

There are 2 ways to go about this. The fully blown application (with a full set of configuration) and a simpler Webhooks approach. This ticket is for the webhooks approach - which can be set up reasonably simply. See:

• Slack.WebHooks (a simple implementation, not recently updated)
• Sending messages using incoming webhooks (Slack documentation)

When Presence starts to add further social features - checking notifications, reading channels, replying to posts, and doing so on behalf of a user, then it will be time to develop a full application. See: SlackNet (C# library, updated recently) as a potential support library to work with.

Implementation notes

• Implement basic posting with incoming webhooks
• Implement basic thread posting
• Implement basic image support
• Create a follow on ticket for a more complete solution with SlackNet

Testing

There's a webhook url for posting to the #integration-testing channel on Presence post, test, and support

Blocked by: #19

[github-actions]

Status: Ready to merge ✔️

Issues blocking this PR:

• Support multiple identities on the same social network #19 ✔️

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This comment was automatically written by the Blocking Issues bot, and this PR will be monitored for further progress.

[instantiator]

Documentation links

• https://api.slack.com/messaging/webhooks
• https://api.slack.com/surfaces/messages
• https://api.slack.com/reference/surfaces/formatting
• https://api.slack.com/reference/block-kit/blocks