jwt: support RFC 7523 JWT Profile

[SmotrovaLilit]

Current implementation client authentication with jwt is not supported rfc 7523 Client Authentication.

In https://github.com/golang/oauth2/blob/master/jwt/jwt.go token request likes as

 POST /token.oauth2 HTTP/1.1
 Host: as.example.com
 Content-Type: application/x-www-form-urlencoded

 grant_type=urn%3Aietf%3Aparams%3Aoauth%3Agrant-type%3Ajwt-bearer
 &assertion=eyJhbGciOiJFUzI1NiJ9....

In rfc 7523 https://tools.ietf.org/html/rfc7523#section-3 token request likes as:

POST /token.oauth2
     Host: as.example.com
     Content-Type: application/x-www-form-urlencoded

     grant_type=authorization_code&
     code=n0esc3NRze7LTCu7iYzS6a5acc3f0ogp4&
     client_assertion_type=urn%3Aietf%3Aparams%3Aoauth%3A
     client-assertion-type%3Ajwt-bearer&
     client_assertion=eyJhbGciOiJSUzI1NiIsImtpZCI6IjIyIn0.
     eyJpc3Mi[...omitted for brevity...].
     cC4hiUPo[...omitted for brevity...]

Do you plan to implement rfc7523?

[jordaniversen]

Any update on this?