fix!: use ProviderConfig::get_slug() for registry keys

[justlevine]

What

Changes the ProviderRegistery to use ProviderConfig::get_slug() for the provider class keys. This ensures the code will continue to work even if the slug of a ProviderConfig gets changed in the future.

Why

Fixes #56 (caused by the slug for OAuth2 Generic changing in v0.0.7)

How

Testing Instructions

Additional Info

Checklist:

• [x] My code is tested to the best of my abilities.
• [x] My code follows the WordPress Coding Standards.
• [x] My code has proper inline documentation.
• [ ] I have added unit tests to verify the code works as intended.
• [x] I included the relevant changes in CHANGELOG.md

[coveralls]

Coverage: 83.741%. Remained the same when pulling cea2a38d28e9a2d326f04d4eea6900959d4ef8fe on justlevine:fix/provider-registry-keys into dbb2aba28a61d7aff6e75633cd486dee11eea3a6 on AxeWP:develop.

[justlevine]

Need to either find a way to avoid the type name change altogether or make the generated names more semantic before merging 🤔