dev: add new `actions/filters to Client class

[justlevine]

What

This PR adds the following Actions/Filters to the plugin:

• graphql_login_authenticated_user_data: Filters the user data returned by the Provider, before it is used to match/create the user.
• graphql_login_after_authenticate: Triggers after a user is authenticated by the Provider.
• graphql_login_pre_get_user_from_data: Lets you shortcircut the user-matching logic.
• graphql_login_get_user_from_data: Fires after the WordPress User has been matched.
• graphql_login_create_user_data : Filters the provider data used to create a new WordPress user when no match is found.

Additionally: this PR fixes an issue with graphql_login_after_successful_login, where the WP_User was being passed twice, instead of the user data from the provider. Props @martinowren in #83

Why

These actions/filters will let you you modify a Provider's auth behavior without requiring you to create a custom ProviderConfig class, lowering the barrier to entry and making it easier to use the Generic Oauth2 Client (and future generic types 🤫 ).

Inspired by @martinowren over on Slack 🙏

How

Testing Instructions

Additional Info

Supersedes #83

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.
• [x] I have added unit tests to verify the code works as intended.
• [x] I included the relevant changes in CHANGELOG.md

[coveralls]

coverage: 83.896% (+0.1%) from 83.799% when pulling 6308990af41ea0df686b99e48311a4a564d31047 on justlevine:dev/client-actions-and-filters into 28a298303b23c0c64b4cf84bd0a16a4d78c8f0d4 on AxeWP:develop.