Improve JWT token issuing

[sagikazarmark]

• Make token key encoding configurable
  • https://github.com/distribution/distribution/blob/3ddd142339615c1e13550f021af4eebf1172fa95/docs/content/spec/auth/jwt.md#verifying-the-token
  • https://github.com/distribution/distribution/blob/3ddd142339615c1e13550f021af4eebf1172fa95/registry/auth/token/token.go#L175
• Embed JWK if necessary
• Embed KID if necessary/possible
• Drop libtrust library (if possible)