Add basic CLI

[sd109]

Currently supports host and port args (which can also be set by S3_ACTIVE_STORAGE_{HOST,PORT} env vars. Is it worth adding some kind of --debug flag for setting log level instead of falling back to RUST_LOG?

[sd109]

Example of current cli help output:

S3 Active Storage Proxy command line interface

Usage: s3-active-storage [OPTIONS]

Options:
      --host <HOST>  [env: S3_ACTIVE_STORAGE_HOST=] [default: 0.0.0.0]
      --port <PORT>  [env: S3_ACTIVE_STORAGE_PORT=] [default: 8080]
  -h, --help         Print help

[sd109]

Addresses #14

[sd109]

Newer CLI help output with added doc strings:

S3 Active Storage Proxy command line interface

Usage: s3-active-storage [OPTIONS]

Options:
      --host <HOST>  The IP address on which the proxy should listen [env: S3_ACTIVE_STORAGE_HOST=] [default: 0.0.0.0]
      --port <PORT>  The port to which the proxy should bind [env: S3_ACTIVE_STORAGE_PORT=] [default: 8080]
  -h, --help         Print help