Add cmd line parser

[sebastianwebber]

This PR 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.

❯ cargo run -- --help  
    Finished dev [unoptimized + debuginfo] target(s) in 0.27s
     Running `target/debug/pgcat --help`
PgCat: Nextgen PostgreSQL Pooler

Usage: pgcat [OPTIONS] [CONFIG_FILE]

Arguments:
  [CONFIG_FILE]  [env: CONFIG_FILE=] [default: pgcat.toml]

Options:
  -l, --log-level <LOG_LEVEL>  [env: LOG_LEVEL=] [default: INFO]
  -h, --help                   Print help
  -V, --version                Print version

[levkk]

Love it, thank you!