Save the session in the AuthClient

[martin-g]

What kind of change does this PR introduce?

New feature (idea for improvement):
The Session returned by the login_*** methods is stored in the AuthClient itself.
This way the AuthClient could use the correct access_token in get_user(), get_settings(), etc. without requiring the developer to provide the access token as a method parameter. Same for the refresh_token in refresh_session() method.

What is the current behavior?

The Session is returned from the login_***() methods and it is responsibility of the user application to pass the correct token where needed.

What is the new behavior?

The Session is still returned by the login_**() methods and the user application may use it if needed.
But the AuthClient itself also keeps a handle to the last authenticated session and use its tokens appropriately where needed.

Additional context

My idea is inspired by floris-xlx/supabase_rs#42
There I want to authenticate the user via supabase.auth().login_***() and start doing supabase.insert()/update()/select()/delete(). By keeping a reference to the Session the insert/update/... operations could use the correct bearer depending on whether the user is authenticated or not:
https://github.com/floris-xlx/supabase_rs/pull/42/files#diff-b1a35a68f14e696205874893c07fd24fdb88882b47c23cc0e0c80a30c7d53759R432-R437

[martin-g]

I didn't do the changes in get_user() and refresh_session() before I get your feedback.
Making these changes will be API breaks!