Load external denylist entries via ServiceLoader

[erawhctim]

Summary

Allow consumers to supply their own DenyListedEntrys via a ServiceLoader interface. Fixes #241.

Open to suggestions on any of the names, or if there's a preferred use of ServiceLoader (it's new to me!)

Requirements (place an x in each [ ])

• [x] I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• [x] I've read and agree to the Code of Conduct.

The following point can be removed after setting up CI (such as Travis) with coverage reports (such as Codecov)

• [x] I've written tests to cover the new code and functionality included in this PR.

The following point can be removed after setting up a CLA reporting tool such as cla-assistant.io

• [x] I've read, agree to, and signed the Contributor License Agreement (CLA).

[erawhctim]

when running spotlessApply locally, I hit an error that I'm not sure what to do with. Subsequent spotless runs seemed to want to delete various files in my staging area 🤔 Have you run into this before?

> Task :spotlessKotlinExternalApply FAILED

FAILURE: Build failed with an exception.

* What went wrong:
Execution failed for task ':spotlessKotlinExternalApply'.
> java.nio.file.FileAlreadyExistsException: /Users/mitchware/Desktop/Workspace/erawhctim-slack-lints/build/spotless/spotlessKotlinExternal/slack-lint-checks/src/test/java/slack/lint/denylistedapis/TestEntriesLoader.kt -> /Users/mitchware/Desktop/Workspace/erawhctim-slack-lints/slack-lint-checks/src/test/java/slack/lint/denylistedapis/TestEntriesLoader.kt

Same issue happened with the DenyListedEntryLoader interface.

[ZacSweers]

For the spotless issue, just re-run it again without configuration cache. It's a weird issue with spotless that I don't really understand :/