Verify nginx config in functional tests

[kate-osborn]

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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?