Add cmd line parser (#512)

This commit adds the clap library and configures the necessary args to
parse from the command line,  expanding the current option of a single
file and adding support for environment variables.

Signed-off-by: Sebastian Webber <sebastian@swebber.me>

src/cmd_args.rs

@@ -0,0 +1,17 @@
+use clap::Parser;
+use log::LevelFilter;
+
+/// PgCat: Nextgen PostgreSQL Pooler
+#[derive(Parser, Debug)]
+#[command(author, version, about, long_about = None)]
+pub(crate) struct Args {
+    #[arg(default_value_t = String::from("pgcat.toml"), env)]
+    pub config_file: String,
+
+    #[arg(short, long, default_value_t = LevelFilter::Info, env)]
+    pub log_level: log::LevelFilter,
+}
+
+pub(crate) fn parse() -> Args {
+    return Args::parse();
+}