Add IngressMTLS policy support

[lucacome]

Proposed changes

Add IngressMTLS policy support

Checklist

Before creating a PR, run through this checklist and mark each as complete.

• I have read the CONTRIBUTING doc
• I have added tests that prove my fix is effective or that my feature works
• I have checked that all unit tests pass after adding my changes
• I have updated necessary documentation
• I have rebased my branch onto master
• I will ensure my PR is targeting the master branch and pulling from my branch from my own fork

[Dean-Coakley]

LGTM.

Just docs comments.

[pleshakov]

@lucacome
Please see my comments and suggestions. Mostly docs.

[pleshakov]

@lucacome
We also need to capture how to propagate client cert info to the backend. Something like below:

  - path: /coffee
    action:
      proxy:
        upstream: coffee
        requestHeaders:
          set:
          - name: s-dn
            value: ${ssl_client_s_dn}

I will add some documentation how to do that (and how to pass JWT claims/JOSE headers of JWT policy) as part of the doc update for this feature #1158

[pleshakov]

@lucacome the changes look good! I noticed a small problem. Could you fix before merging?