[SDK-2811] Replace request with axios

[frederikprijck]

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Description

Request has been deprecated for a while now, see request/request#3142.
This PR replaces request with axios.

Request was only used in the _getAccessToken method on the Auth0Strategy which is marked as TODO: Obsolete.
I agree with the fact that this method should be marked obsolete and, ideally, get removed in a future version. I properly marked the method as obsolete.

References

#137

Testing

Calling _getAccessToken still gives you the corresponding tokens in the callback.

• This change adds test coverage for new/changed/fixed functionality

Checklist

• I have added documentation for new/changed functionality in this PR or in auth0.com/docs
• All active GitHub checks for tests, formatting, and security are passing
• The correct base branch is being used, if not master