Add Discord webhook exposed key handling

[gjcthinkst]

Proposed changes

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

Types of changes

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

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

Checklist

• 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 Discord 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 Discord messages displayed correctly.

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

[wleightond]

Moving away from typing.{List,Tuple,Dict} to their builtin equivalents will apply to other PRs as well