Verify nginx config in functional tests

[kate-osborn]

As a maintainer of NGF
I want to verify that our NGINX config is what we expect for every functional test we have
So that do not just test for the correct behavior from NGF as a whole, but we also test that we are setting the right configuration for the behavior we expect.

Acceptance

• NGINX configuration verification is added as a feature to our testing framework.
  • Assert that a directive is present in a given location. E.g. client_max_body size is set to 2MB in the server context.
• ClientSettingsPolicy functional test is updated to verify NGINX configuration applied.

Dev Notes

• Some tool might be available to print out NGINX config in JSON format. Crossplane?

[mpstefan]

What do we want to test here, specifically? Are we going to check that the NGINX matches exactly what we expect? Is this valuable if we have to update our tests after each change that affects configuration?