Enhancement: selectively manage secrets separately (not all or nothing)

[ja20222]

UPDATE LINKS AFTER #807 is merged

All configuration files, including Secret are replaced during reconfiguration. See https://github.com/nginxinc/nginx-kubernetes-gateway/blob/c02f7991e206a1562c803a19306f855d4308cc86/internal/nginx/file/manager.go#L88

Optimize, so that you don't need to rewrite configuration files that have not been changed.

[sjberman]

Closing this since this work will no longer be relevant with the 2.0 release.