add Send/Sync tests

[notoriaga]

Figured out a way to show/test an issue I've talked about before

error: future cannot be sent between threads safely
   --> crates/esthri/tests/integration/send_sync.rs:77:22
    |
77  |           require_send(&$value);
    |                        ^^^^^^^ future is not `Send`
...
115 | / async_assert_fn!(
116 | |     esthri::upload(_, &str, &str, &Path, opts::EsthriPutOptParams): Send & !Sync & !Unpin
117 | | );
    | |_- in this macro invocation
    |
    = help: the trait `Sync` is not implemented for `(dyn Stream<Item = Result<esthri_internals::hyper::body::Bytes, std::io::Error>> + std::marker::Send + 'static)`
note: required by a bound in `require_send`
   --> crates/esthri/tests/integration/send_sync.rs:41:20
    |
41  | fn require_send<T: Send>(_t: &T) {}
    |                    ^^^^ required by this bound in `require_send`
    = note: this error originates in the macro `async_assert_fn_send` which comes from the expansion of the macro `async_assert_fn` (in Nightly builds, run with -Z macro-backtrace for more info)

warning: `esthri` (test "integration") generated 2 warnings

namely that the futures returned by some of the functions aren't Send which can make them more challenging to work with