Wish: expose serve() & co for testing

[brainstorm]

After seeing your comment in datenlord/s3-server#108 (comment) and given that I'm getting cargo audit errors by using datenlord/s3-server on my CI, which pulls Rusoto (deprecated), I've considered switching to your crate to substitute a test that is using s3-server from @datenlord.

All I'd need is to have the serve() method you have in server.rs and perhaps some accessory ones to just get/put some example file(s) on my small testsuite, but currently the crate only seems to be available as a standalone binary instead of lib:

% cargo build
    Updating crates.io index
warning: s3d-test-example v0.1.0 (/Users/rvalls/dev/umccr/s3d-test-example) ignoring invalid dependency `s3d` which is missing a lib target
   Compiling s3d-test-example v0.1.0 (/Users/rvalls/dev/umccr/s3d-test-example)
    Finished dev [unoptimized + debuginfo] target(s) in 1.36s