feat(formsg): clone repo on webhook trigger from forms

[seaerchin]

Problem

See here

Solution

1. add an endpoint at /formsg/clone-repo
2. add handler to clone at that endpoint based on the form's reply. The form has validation build in (email domain must be .open.gov.sg and must verify email + repo is a required field)

New env vars

• SITE_CLONE_FORM_KEY: used to decrypt the form sg submissions (already added to 1PW on both staging and prod env vars; not yet added to staging EB yet)

Notes

this was tested locally through updating the EFS_VOL_PATH + originUrl (use https instead of ssh as i was facing auth issues) and found working

[seaerchin]

This PR has quite a number of infra moving parts - eg.

1. formsg
2. efs
3. be

Could we do a quick sanity check testing on stg before release? since this is internal use using post deploy testing is also fine, thoughts?

tbh, i think the only difference between testing on staging vs not, is that the clone step takes place in a different environment (with all of the annoyances that entails); i think i'll just do post-deploy cos it's not open to public anyway.