Adds SP-initiated LogoutRequests

[escherize]

The basic flow of a SAML logout is:

1. A SSO SAML User clicks Sign Out.
2. Service Provider issues a redirect to the client with a LogoutRequest to the Identity Provider.
3. Client forwards the request to the Identity Provider.
4. Identity Provider logs the user out + redirects client back to Service Provider with a LogoutResponse.
5. Service Provider clears the user's session, responds to the client with a redirect to the home page.

This adds logout-redirect-location and idp-logout-redirect-response, which are used to send a LogoutRequest to an Identity Provider. Some users of this library (like Metabase) prefer to send the redirect url in a POST body, and let the client initiate the redirect, and that's what logout-redirect-location is for.

[codecov]

Codecov Report

Attention: 14 lines in your changes are missing coverage. Please review.

Comparison is base (a1461ad) 75.30% compared to head (2ad374e) 75.03%.

Files	Patch %	Lines
src/saml20_clj/sp/request.clj	77.96%	2 Missing and 11 partials ⚠️
src/saml20_clj/sp/metadata.clj	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #75      +/-   ##
==========================================
- Coverage   75.30%   75.03%   -0.27%     
==========================================
  Files          10       10              
  Lines         656      693      +37     
  Branches       90      101      +11     
==========================================
+ Hits          494      520      +26     
  Misses         72       72              
- Partials       90      101      +11     

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[camsaul]

amazing

[escherize]

thanks

[camsaul]

works for me

[camsaul]

Might be worth adding some info about this in the README.md so people can see at a glance how to configure SP-initiated logout stuff

[dpsutton]

approving because Cam approved but a small change invalidated his approval