feat: Standardize environment variables for database connection.

[EdwinBetanc0urt]

Add DB_ENGINE environment variable, and unify alls DB_* redundant environment variables.

Additional context The previous method of configuring databases still works, to provide backward compatibility, but priority is given to this proposed approach. In the future (in a major version), the previous method should be removed.

fixes https://github.com/NginxProxyManager/nginx-proxy-manager/issues/4677

[nginxproxymanagerci[bot]]

Docker Image for build 10 is available on DockerHub:

nginxproxymanager/nginx-proxy-manager-dev:pr-4678

[!NOTE] Ensure you backup your NPM instance before testing this image! Especially if there are database changes. This is a different docker image namespace than the official image.

[!WARNING] Changes and additions to DNS Providers require verification by at least 2 members of the community!