Define Modsecurity Snippet via ConfigMap

[MRoci]

What this PR does / why we need it:
It would be handy to have a key in ConfigMap to define custom ModSecurity Rules shared across Ingresses, but now it's possible only with the annotation nginx.ingress.kubernetes.io/modsecurity-snippet.
This PR adds modsecurity-snippet key, whose loading is subordinated to what is defined by the annotation

[fejta-bot]

Issues go stale after 90d of inactivity.
Mark the issue as fresh with /remove-lifecycle stale.
Stale issues rot after an additional 30d of inactivity and eventually close.

If this issue is safe to close now please do so with /close.

Send feedback to sig-testing, kubernetes/test-infra and/or fejta.
/lifecycle stale

[aledbf]

/ok-to-test

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@9ecec0d). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master    #4087   +/-   ##
=========================================
  Coverage          ?   58.74%           
=========================================
  Files             ?       88           
  Lines             ?     7116           
  Branches          ?        0           
=========================================
  Hits              ?     4180           
  Misses            ?     2490           
  Partials          ?      446

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 9ecec0d...1ee081c. Read the comment docs.

[aledbf]

/retest

[theoretick]

👋 Hello! I was wondering if there was any updates on this PR and/or is there anything I could do to help it along? This would be a massive addition as without custom snippets we cannot enable the ruleset in blocking mode, customize logging, add custom rules, or a number of other critical changes.

Happy to help push this one over the finish line if any assistance is needed!

[MRoci]

wave Hello! I was wondering if there was any updates on this PR and/or is there anything I could do to help it along? This would be a massive addition as without custom snippets we cannot enable the ruleset in blocking mode, customize logging, add custom rules, or a number of other critical changes.

Happy to help push this one over the finish line if any assistance is needed!

I've rebased, asap i'll look into any failiing test

[aledbf]

/lgtm

[aledbf]

@MRoci thanks!

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: aledbf, MRoci

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• OWNERS [aledbf]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment