feat: enhanced user matcher

[coltenkrauter]

Description

Add enhanced user matching logic that supports user supplied matching (so the action accepts user-mappings like, octocat:slackcat where the Github user can be matched to the Slack user.

Pending investigation, this may help with #34.

Type of Change

• New feature
• Bug fix
• Chore
• Configuration
• Documentation
• Refactoring
• Hotfix
• Security
• GitFlow

Testing

• Executed locally
• Unit tests
• Integration tests
• Load tests
• PR Status checks
• Workflow execution (against non-default branch)
• Other (explain)
• Not worth it (explain)
• Not applicable

Checklist

[Not required to be completed, just check off what you've done.]

• Commits are reasonably sized and the commit message describes the changes well.
• I have made corresponding changes to the documentation.
• I have added unit tests for changes to business logic.
• I have emitted metrics so our team can monitor the health of this system related to these changes.
• I have added logs to ensure debugging is straight-forward when issues arise.
• I have considered failure modes (e.g. major dependency becomes unavailable, bad data passed in).
• I have added a comment to this PR to discuss any concerns/risks of these changes. This might include effects on performance, new dependencies, changes in behavior, or difficult things to test.
• I have considered whether some of these changes should go into a shared package to increase code reusability.