Cleanup Auth0 class constructor for clarification and better defaults

[joshcanhelp]

Changes

Potentially breaking changes:

• Add ENV fallbacks for required settings:
  • AUTH0_DOMAIN will be used if the domain configuration key is empty
  • AUTH0_CLIENT_ID will be used if the client_id configuration key is empty
  • AUTH0_REDIRECT_URI will be used if the redirect_uri configuration key is empty
• Change skip_userinfo default to true. This means that applications no setting this value will get the user identity from the ID token rather than an extra HTTP call to the /userinfo endpoint.
• Remove unused debugging mode and public Auth0->setDebugger() method
• Base64 decode Client Secret in the constructor to be used as-is within the class

Testing

• This change adds test coverage
• This change has been tested on the latest version of PHP

Checklist

• All existing and new tests complete without errors.
• The correct base branch is being used.

[damieng]

Is the removal of setDebugger a breaking change?

[joshcanhelp]

@damieng - Potentially, yes. This is for a major release so we're ok. I added more specifics in the PR description.

[github-actions]

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