add Verification Certificate to SAML source

[matthias-bruhse]

Fixes part of #596.

This only adds the Verification Certificate option to the SAML source. I did not have time to fully implement the Icon setting as well, with it being more complicated due to the different API endpoints for uploads and URLs and therefore input options. I do have a branch in my fork that allows setting the icon via URL which is my use case. Might be useful as a starting point, if you get around to work on it.

[codecov]

Codecov Report

Attention: Patch coverage is 70.00000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 79.29%. Comparing base (7e6942e) to head (75b77b9).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
internal/provider/resource_source_saml.go	70.00%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #615      +/-   ##
==========================================
- Coverage   79.32%   79.29%   -0.04%     
==========================================
  Files         101      101              
  Lines       11474    11484      +10     
==========================================
+ Hits         9102     9106       +4     
- Misses       1901     1905       +4     
- Partials      471      473       +2     

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