Support `kid` to locate the key in a JSON Web Key Set (JWKS)

[dumbbell]

In encode/{3,4}, it's possible to annotate the key with it key ID using a tuple:

jwt:encode(Alg, ClaimsSet, {<<"MyKeyId">>, Key}).

The kid attribute will be added to the token header.

In decode/{2,3}, it's possible to pass an encoded (JSON as a binary) or decoded (JSON as a map) JWKS:

jwt:decode(Token, JWKS).

jwt will call jwk to decode (if necessary) and select the appropriate key in the JWKS during the get_key step, as the kid was obtained previously during the decode_jwt step.

As part of that, new type specs were defined:

• key()
• key_id()

Care has been taken to not break the API in the process.

[artemeff]

thanks! sorry for delay :(

[dumbbell]

No problem :-) Thank you!