Add possibility to change nginx->workhorse proxy buffer size

What does this MR do?

In cases where Workhorse would return a header that's too big, for example when proxying an already large header, this would fail with a 502 and no "option" to configure this except manually changing the nginx config.

Related issues

Closes #6678 (closed)

Checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion

Required

• Merge Request Title, and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
• MR has a green pipeline on GitLab.com
• Pipeline is green on dev.gitlab.org if the change is touching anything besides documentation or internal cookbooks
• trigger-package has a green pipeline running against latest commit

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
  • Not setting nginx['proxy_custom_buffer_size'] should leave the template as before this branch
  • Setting nginx['proxy_custom_buffer_size'] to a specific value (42k for example) should add a section in /var/opt/gitlab/nginx/conf/gitlab-http.conf under the gitlab_workhorse upstream proxy, which includes this custom buffer size (as per the MR)
• Documentation created/updated
  • Updated the template, will add a Geo troubleshooting section after this is merged as well
• Tests added
• Integration tests added to GitLab QA
• Equivalent MR/issue for the GitLab Chart opened