fix: webhook transient payload in OIDC login flows

[hperl]

This fixes a bug where we did not put the transient payload into the continuity container on OIDC logins, which caused OIDC login flows to lose the transient payload.

Also, this PR adds more webhook tests in general and a handy hooktest package to test webhooks.

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
  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

[jonas-jonas]

Thank you

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.04%. Comparing base (da90502) to head (8b82e67).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3857      +/-   ##
==========================================
+ Coverage   77.98%   78.04%   +0.06%     
==========================================
  Files         359      360       +1     
  Lines       25203    25233      +30     
==========================================
+ Hits        19654    19694      +40     
+ Misses       4041     4033       -8     
+ Partials     1508     1506       -2     

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