Add Slack webhook exposed key handling

[gjcthinkst]

Proposed changes

Add Slack webhook support for exposed API key alerts. While we're in the area also do some refactoring and general improvements for the Slack webhook.

Types of changes

What types of changes does your code introduce to this repository?

• New feature (non-breaking change which adds functionality)

Checklist

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• Lint and unit tests pass locally with my changes (if applicable)
• I have run pre-commit (pre-commit in the repo)
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)
• Linked to the relevant github issue or github discussion

Further comments

I tested the changes to the Slack webhook by creating a new AWS API key token, triggering it normally and manually creating an exposed key alert with CURL (with 25 Dec as the exposed time) and checked that all three Slack messages displayed correctly.

An example of the current (before this change) Slack message format is shown below.

[thinkst-tom]

@gjcthinkst Seeing as we're refactoring here, do you perhaps think that this might be a good time to also change to using the newer block format for the webhook payload? Slack has marked attachments (which are used in the current webhook payload) as a legacy feature.

[gjcthinkst]

@gjcthinkst Seeing as we're refactoring here, do you perhaps think that this might be a good time to also change to using the newer block format for the webhook payload? Slack has marked attachments (which are used in the current webhook payload) as a legacy feature.

@thinkst-tom Good suggestion, thanks! I've updated the code to use the new block format.

[thinkst-tom]

Thanks @gjcthinkst this looks great!