Add support for encryption (RFC7516)

[lggomez]

Migrated from https://github.com/dgrijalva/jwt-go/issues/346:

maxshifrin commented on Jul 30, 2019

It would be great if this library could also support payload encryption (https://tools.ietf.org/html/rfc7516)

It would greatly help in case sensitive information needs to be stored inside the JWT token, and offload the need to encrypt it in every application separately