Support slack events about file

[andyhaskell]

This PR adds support for deserializing file_change, file_deleted, file_shared, and file_unshared events from the Slack Events API

Pull Request Guidelines

These are recommendations for pull requests. They are strictly guidelines to help manage expectations.

PR preparation

Run make pr-prep from the root of the repository to run formatting, linting and tests.

Should this be an issue instead

• [ ] is it a convenience method? (no new functionality, streamlines some use case)
• [ ] exposes a previously private type, const, method, etc.
• [ ] is it application specific (caching, retry logic, rate limiting, etc)
• [ ] is it performance related.

API changes

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• no tests, if you're adding to the API include at least a single test of the happy case.
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• dependency changes. updates are okay. adding/removing need justification.

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• in library cache for users. caches are use case specific.
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[andyhaskell]

@kanata2 No problem! Btw it looks like in the 1.17 tests, TestGetRTMInfo is failing. I haven't used this CI system before, so I was wondering, is there a way to restart that build? At least the wording there makes it sound like a transient issue

[kanata2]

Actually, this test is so fraky... I understood test failure is not related to this changes, so I'll merge.