Ensure NGINX reload actually happens

[kate-osborn]

When we reload NGINX, we do not check whether the reload succeeds. We just sleep for a second to wait for the new workers to spin up. We need to verify that a reload was successful and return an error if it fails. Additionally, we should retry on failure.

Acceptance:

• When a NGINX reload occurs, NKG will wait for the reload to finish, and if an error occurs, the error is reported to the user.
• When NGINX fails to reload, NKG will retry reloading NGINX.

This work should resolve the following FIXME: https://github.com/nginxinc/nginx-kubernetes-gateway/blob/b00216eeac0f8edfb054d45ad073c9c004788762/internal/nginx/runtime/manager.go#L52

[mpstefan]

Logs could be the solution to this problem.