feat(provider): add signicat idp provider

[dahlmo]

Adds Signicat as a supported provider. Signicat is an OpenID-compliant Digital Identity Platform, commonly used in Norway as an integration partner towards "BankID" which is a secure authentication solution used by banks, government and in later years also by businesses to let people authenticate with their SSN (Social Security Number), personal password and OTP.

Related issue(s)

None

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
  confirm that I got the approval (please contact
  security@ory.sh) from the maintainers to push
  the changes.
• I have added tests that prove my fix is effective or that my feature
  works.
• I have added or changed the documentation.

Further Comments

This feature (Signicat provider) is currently used by https://otto.no/ (car sharing platform) in production in our self-hosted Ory Kratos (via Ory Helm Charts). Since this is not merged into Kratos, we're building our own image from the source with the modifications in this PR in it.

I have never coded in Go before I created this PR, so please go easy on me. I hope to get some help with the tests, but if not possible I'll try to set aside some time and teach myself some more Go.

[codecov]

Codecov Report

Merging #3587 (8e19e6f) into master (e8b92c1) will decrease coverage by 0.19%.
Report is 24 commits behind head on master.
The diff coverage is 74.65%.

❗ Current head 8e19e6f differs from pull request most recent head 7254d3e. Consider uploading reports for the commit 7254d3e to get more accurate results

@@            Coverage Diff             @@
##           master    #3587      +/-   ##
==========================================
- Coverage   78.15%   77.97%   -0.19%     
==========================================
  Files         341      342       +1     
  Lines       22861    22916      +55     
==========================================
  Hits        17868    17868              
- Misses       3653     3708      +55     
  Partials     1340     1340              

Files	Coverage Δ
courier/handler.go	90.00% <ø> (ø)
driver/config/config.go	83.28% <ø> (ø)
identity/identity.go	87.96% <100.00%> (ø)
identity/manager.go	75.11% <100.00%> (ø)
identity/test/pool.go	99.55% <100.00%> (ø)
schema/handler.go	83.52% <ø> (ø)
selfservice/flow/login/hook.go	88.55% <100.00%> (ø)
selfservice/hook/show_verification_ui.go	100.00% <ø> (ø)
selfservice/hook/web_hook.go	79.65% <100.00%> (ø)
selfservice/strategy/oidc/provider.go	100.00% <ø> (ø)
... and 11 more