Support HTTP-POST-SimpleSign binding

[BotoX]

Hi,

pysaml2 does not seem to support the SimpleSign version of the HTTP POST SAML2 binding urn:oasis:names:tc:SAML:2.0:bindings:HTTP-POST-SimpleSign,

saml2.SAMLError: Unknown binding type: urn:oasis:names:tc:SAML:2.0:bindings:HTTP-POST-SimpleSign

https://docs.oasis-open.org/security/saml/Post2.0/sstc-saml-binding-simplesign.html

I was hoping to use SATOSA to connect our Keycloak with the eduID.at and eduGAIN Federation.
Actually the project is almost finished but can not proceed further until this SAML binding is supported due to some SPs relying on it.

I can see myself implementing support for this binding if it's not too much work.
Would appreciate any help though, thank you.

[BotoX]

I wrote some code here:
BotoX@bd7384e
to fix the issue for our usecase:

• SATOSA IdP proxy in eduID.at/eduGAIN
• SP imoox.at with Shibboleth SSO SP
• Sends AuthnRequest with: ProtocolBinding="urn:oasis:names:tc:SAML:2.0:bindings:HTTP-POST-SimpleSign" to our SATOSA