Make SAML binding configurable via idp.saml_binding

[mloesch]

Currently the SAML redirect binding is hardcoded in the IDP configuration. However, our SP only works with the HTTP POST binding.

This PR makes the binding type configurable via a property. I tested this with the HTTP POST binding (urn:oasis:names:tc:SAML:2.0:bindings:HTTP-POST) against our SAML SP and it works fine.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.25%. Comparing base (71528b3) to head (215b52f).
Report is 23 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##             master      #86   +/-   ##
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  Coverage     91.25%   91.25%           
  Complexity      165      165           
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  Files            30       30           
  Lines           583      583           
  Branches         17       17           
=========================================
  Hits            532      532           
  Misses           37       37           
  Partials         14       14           

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[oharsta]

Nice addition. Merged (and added urn:oasis:names:tc:SAML:attribute:subject-id to the default attributes).