Add configurable data plane log level

[bjee19]

Add Nginx error log level to NginxProxy.

Problem: Users would like to assign a log level for the data plane.

Solution: Add Nginx error log level to NginxProxy which allows users to set the error log level for Nginx.

Testing: Unit tests and manual testing.

Manual testing included

• Validating the enum values in the new ErrorLevel field in the NginxProxy resource.
• Deploying NGF, setting up the cafe-app, deploying an NginxProxy resource, and testing each of the possible log levels and checking that the nginx conf (the main.conf file and running nginx -T) in the nginx container was updated correctly.
• Checked that if the error level was not set in the NginxProxy resource, the nginx conf would be correctly updated back to the default of info log level.
• Tested that if a log level was set to something "beneath" notice such as the error level, after making changes which would result in notice log levels being outputted (such as deleting and re-applying cafe.yaml in the cafe example), no notice log levels are outputted.
• nginx -T correctly shows the main.conf configuration file being included in the nginx configuration.
• ps aux |grep nginx when inside the nginx container correctly showed nginx-debug processes.
• Deployed the cafe example and deleted it a few times and NGF still worked properly with no error logs in either container.

Closes #901

Checklist

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

Release notes

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