fix type specs and doc

[woylie]

This fixes three issues I encountered.

1. The docs for the Joken.Hooks.RequiredClaims had an example which wrapped the required claims in a keyword list, but the hook required just a plain list. I changed the docs. However, the Joken.Hooks behaviour asks for a keyword list as the first argument for all callbacks, so it would probably be better to change the implementation, although that would be a breaking change. If you prefer that, let me know and I'll change it.
2. Joken.Jwks requires you to set the default signer to nil. If you call Joken.verify_and_validate/5 etc. directly and pass nil as a signer, you will get a dialyzer error. I changed the type specs of all those functions to allow nil as signer.
3. You can pass options along with the hook module name, which wasn't reflected in the type specs and caused dialyzer errors.

[victorolinasc]

Awesome @woylie ! Thanks a lot for your work!

I don't think we should add a breaking change... it was my mistake but people should be already depending on it.

I still plan to release a new major version but haven't had the time to code about a crypto only signer dep. Let's live with this incompatibility for the time being.

Once again thanks for your work. I'll merge this and some other deps update and release a new version soon.