Support multiple peer files from environment variable

[ceresnam]

This change enables specifying multiple WireGuard configuration files for the extract_names_config_files option using the PROMETHEUS_WIREGUARD_EXPORTER_CONFIG_FILE_NAMES environment variable.

It utilizes std::env::split_paths to automatically handle the correct OS-specific path list separator (: for Linux/Unix, ; for Windows).

Example (Environment Variable):

PROMETHEUS_WIREGUARD_EXPORTER_CONFIG_FILE_NAMES="/tmp/wg1/conf:/tmp/wg2.conf" prometheus_wireguard_exporter

This correctly parses as:

[INFO] using options: Options { extract_names_config_files: Some(["/tmp/wg1/conf", "/tmp/wg2.conf"]) }

This complements the existing command-line functionality:

prometheus_wireguard_exporter -n /tmp/wg1/conf /tmp/wg2.conf

Which also parses as:

[INFO] using options: Options { extract_names_config_files: Some(["/tmp/wg1/conf", "/tmp/wg2.conf"]) }