Implement the different client authentication mechanisms about simpleidentityserver

[thabart]

At the moment only three authentication methods are supported : client_secret_basic, client_secret_post

The following ones need to be supported : client_secret_jwt, private_key_jwt

[thabart]

To support the "client_secret_jwt" we need to support this part : https://tools.ietf.org/html/draft-ietf-jose-json-web-encryption-40#appendix-A.2

The encryption mechanism (alg : RSA1_5 & enc : A128CBC_HS256) is now implemented in the "testing project". We also need to support the algorithms (https://tools.ietf.org/html/rfc7518#page-12):
==> RSA-OAEP : use OAEP
==> RSA-OAEP-256 : use OAEP & SHA256 & MGF1

And also the following encryptions methodologies (https://tools.ietf.org/html/rfc7518#page-22) :
==> A192CBC-HS384 : HMAC using SHA-256
==> A256CBC-HS512 : HMAC using SHA-512

[thabart]

Validate the Client assertion based on this RFC : https://tools.ietf.org/html/draft-ietf-oauth-jwt-bearer-12#section-3

[thabart]

TODO :
=> Write an another UT to test "client_secret_jwt"
=> How to test the issuer value ? Maybe the issuer is the client id ?
=> How to use the parameter "authentication_client_endpt"
=> For the encryption the "client_secret" should be used as the HMAC key

Good def
client_secret_jwt: the client uses the JWT Assertion profile with a symmetric secret issued by the server
private_key_jwt: the client uses the JWT Assertion profile with its own private key