extension: net: Refactor, fix and improve network extensions

- Rename extensions with "net-*" prefix
- Put the extensions into their own folder
- Split off time sync packages into their own extensions to be able to be used separately
- Put their config files into directories instead of using inline `cat <<- EOF >`
- Move some other NetworkManager related stuff into the extension
- Remove unneeded steps
- Install iproute2 by default on all images (for the `ip` command)

lib/functions/general/extensions.sh

@@ -476,6 +476,14 @@ function enable_extension() {
 			extension_dir="${extension_base_path}" # this is misleading. only directory-based extensions should have this.
 			extension_file="${extension_floating_file}"
 			break
+		else
+			# Search for the extension file in any subdirectory
+			extension_file=$(find "${extension_base_path}" -type f -name "${extension_name}.sh" | head -n 1) # Example format: extensions/network/net-network-manager.sh
+			if [[ -n "${extension_file}" ]]; then
+				# Extract extension dir from file, e.g. from "extensions/network/net-network-manager.sh" the dir "extensions/network/" gets extracted
+				extension_dir="${extension_file%/*}"
+				break
+			fi
 		fi
 	done