Add support for user credentials/jwt

[ThomasKranitsas]

Fixes #1

[callmekatootie]

Minor issues but can indeed be merged 👍

[callmekatootie]

Comment: A better configuration in my opinion would have been asking the user itself which approach they are going with - like a type field which can have m2m|user|jwt values instead of us figuring it out.

Also, note that this "check" is also duplicated in the helper function. Too trivial to wrap it in its own method but asking the user itself for the type would have thus helped.

[callmekatootie]

Comment: So the JWT way is to only pass the submission api url and then pass the token each time any method is called? 🤔 hmm... Not sure why the constructor didn't accept the jwt token itself... assuming it is because it allows us to use different tokens for each method...