feat: add custom form_post response writer

[levkohimins]

The current interface FormPostHTMLTemplateProvider only allows rendering a custom form_post template. But there is no way to pass extra template variables to be used in the template.
This pull request adds the ability to assign a custom form_post response writer.

Related Issue or Design Document

Implements a new feature, issue reference #730

Checklist

• I have read the contributing guidelines and signed the CLA.
• I have referenced an issue containing the design document if my change introduces a new feature.
• 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 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 the necessary documentation within the code base (if appropriate).

Further comments

I couldn't manage to pass nancy/test, it claims

pkg:golang/github.com/containerd/[email protected]
7 known vulnerabilities affecting installed version

but I can't figure out which package uses this dependency.

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you all sign our Contributor License Agreement before we can accept your contribution.
1 out of 2 committers have signed the CLA.

✅ levkohimins
❌ Levko Burburas

---

Levko Burburas seems not to be a GitHub user. You need a GitHub account to be able to sign the CLA. If you have already a GitHub account, please add the email address used for this commit to your account.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[levkohimins]

@aeneasr
Hey guys, is there any chance this PR will be reviewed?

[aeneasr]

Hey, open to get this merged - there are just more things that have a higher priority on my list right now :( Was this closed on purpose?

[james-d-elliott]

I suspect maybe because of bc43abc since his may have conflicted and he's going to open another PR. Main reason I opened that was to avoid potential conflicts down the line with unrelated PRs.

[levkohimins]

Hey, open to get this merged - there are just more things that have a higher priority on my list right now

Oh, I thought that this PR went unnoticed.

[mitar]

Please rebase this PR. Currently it contains extra commits/changes.