feat: webhooks support setting client cookies

[brahmlower]

Related issue(s)

None- I've added this without any prior discussion.

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
  [email protected]) 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

Hey folks, it's been a while 👋 Hope you all had a good winter break!

My team realized it would be helpful if we could set an additional cookie via the post-login webhook for some of our users, and so I went ahead and added support for doing just that 😁 This PR is a functional proof of concept.

A webhook response with the following would set a cookie named "foo" with the value "bar":

{
  "cookies": [
    {
      "name": "foo",
      "value": "bar"
    }
  ]
}

As I see it, the only remaining work this branch needs is:

• tests
• docs
• any security checks/guards that you guys think would be appropriate

Before working on that though, I wanted to check in with you guys first to see if this is a feature you'd be interested in merging.

[codecov]

Codecov Report

Attention: Patch coverage is 46.15385% with 28 lines in your changes are missing coverage. Please review.

Project coverage is 77.93%. Comparing base (d01b670) to head (5242e84).
Report is 1 commits behind head on master.

Files	Patch %	Lines
selfservice/hook/web_hook.go	46.15%	27 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3716      +/-   ##
==========================================
- Coverage   77.98%   77.93%   -0.06%     
==========================================
  Files         359      359              
  Lines       25190    25213      +23     
==========================================
+ Hits        19644    19649       +5     
- Misses       4039     4056      +17     
- Partials     1507     1508       +1     

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