feat: cache OIDC providers

[zepatrik]

This change significantly reduces the number of requests to /.well-known/openid-configuration endpoints.

Related issue(s)

Checklist

• I have read the contributing guidelines.
• I have referenced an issue containing the design document if my change
  introduces a new feature.
• I am following the
  contributing code guidelines.
• I have read the security policy.
• I confirm that this pull request does not address a security
  vulnerability. If this pull request addresses a security vulnerability, I
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• I have added tests that prove my fix is effective or that my feature
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Further Comments

[alnr]

IMO a general HTTP client cache would make more sense. It can be used in lots of places.

Based on https://github.com/gregjones/httpcache perhaps.

[codecov]

Codecov Report

Attention: Patch coverage is 85.00000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 78.55%. Comparing base (e6d2d4d) to head (3461443).
Report is 8 commits behind head on master.

Files with missing lines	Patch %	Lines
selfservice/strategy/oidc/strategy.go	86.66%	1 Missing and 1 partial ⚠️
selfservice/strategy/oidc/provider_microsoft.go	50.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4222      +/-   ##
==========================================
+ Coverage   78.52%   78.55%   +0.03%     
==========================================
  Files         380      380              
  Lines       27231    27262      +31     
==========================================
+ Hits        21383    21416      +33     
+ Misses       4224     4223       -1     
+ Partials     1624     1623       -1     

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[alnr]

I don't understand this PR. Where is the caching?

[zepatrik]

Where is the caching?

It's super hard to cache the full client responses correctly, so we added the cache to the go-oidc library instead. There we can do high-level caching.