No support for HTTP POST method at Authorization Endpoint

[cicnavi]

We currently support only HTTP GET method at Authorization Endpoint.

Per OIDC spec for authn request:

Authorization Servers MUST support the use of the HTTP GET and POST methods defined in RFC 2616 [RFC2616] at the Authorization Endpoint. Clients MAY use the HTTP GET or POST methods to send the Authorization Request to the Authorization Server. If using the HTTP GET method, the request parameters are serialized using URI Query String Serialization, per Section 13.1. If using the HTTP POST method, the request parameters are serialized using Form Serialization, per Section 13.2.

[cicnavi]

Note that this is also needed to properly support automatic client registration functionality from OpenID Federation specification.

[cicnavi]

This is addressed in v6 in PR #242.

However, this will have to be refactored in order to support 'request object' from core specification: https://openid.net/specs/openid-connect-core-1_0.html#RequestObject