Feature/retrieve saml metadata via url

[Ewen-Lgy]

Related to ticket open-formulieren/open-forms#1849

I might not have time to close this pull request since it is my last day so here is a summary of what has been done so far and what is left to do.

DONE :

• Created solo models for Ehekernning and Digid
• Created endpoint/urls to generate : Eherkenning, Digid and DiesntCatalogus metadata
• created custom change_form.html admin template so we can have buttons (like history) to generate metadata
• Updated tests from command to models
• Added mixins for generating certificate and fill digid/eherkenning metadata config
• Added tests for metadata endpoints
• Deleted commands for generating metadata

TODO:

• Complete the error message if metadata genreration has failed
• Add more model field verification to generate properly the metadata (especially for OIN and makelaar_id in Eherkenning model)
• Add various dropdowns for different fields
• Update saml2 clients in order to not use EHERKENNING and DIGID settings anymore (only the metadata related settings)
• !! Update the documentation

UNDO:

• commands to generate metadata still have to be there
• Put back metadata commands tests

[codecov]

Codecov Report

Merging #16 (4b9a3d4) into master (34c5a0e) will increase coverage by 2.46%.
The diff coverage is 98.71%.

@@            Coverage Diff             @@
##           master      #16      +/-   ##
==========================================
+ Coverage   93.78%   96.25%   +2.46%     
==========================================
  Files          44       60      +16     
  Lines        2172     2880     +708     
==========================================
+ Hits         2037     2772     +735     
+ Misses        135      108      -27     

Impacted Files	Coverage Δ
digid_eherkenning/mock/views/digid.py	72.54% <ø> (+5.27%)	⬆️
digid_eherkenning/utils.py	92.59% <ø> (ø)
digid_eherkenning/views/digid.py	94.78% <ø> (+0.71%)	⬆️
tests/project/urls.py	100.00% <ø> (ø)
digid_eherkenning/settings.py	92.30% <90.00%> (-7.70%)	⬇️
...t/commands/generate_eherkenning_dienstcatalogus.py	93.54% <92.30%> (+8.64%)	⬆️
digid_eherkenning/management/commands/_base.py	94.93% <94.93%> (ø)
digid_eherkenning/models/digid_metadata_config.py	95.00% <95.00%> (ø)
digid_eherkenning/saml2/base.py	94.90% <96.29%> (+7.24%)	⬆️
..._eherkenning/models/eherkenning_metadata_config.py	97.22% <97.22%> (ø)
... and 31 more

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[sergei-maertens]

• Re-introduce management commands, but instead let them configure/use the solo model config classes. Any CLI flags result in the config class being overridden on the fly. Possibly we can add a --save-config flag to also persist that.
• Regular views/SAML flows etc. should fetch config from the classes rather than from the Django setting
• Optimize the config classes so they always select_related the certificate - this query will always be done and should be avoided.