Add support for async http client

[seemeroland]

The client always uses a blocking http client. Could we add a send_async function that uses reqwest::Client for async support?

Unconditionally using the blocking client also prevents this crate from being compiled for the WASM target: seanmonstar/reqwest#891

[Sytten]

This is also a problem for us

[imricardoramos]

In case anyone else is having this problem, I made a fork here replacing blocking reqwest with async reqwest:
https://github.com/imricardoramos/rudder-sdk-rust-async

[Sytten]

You can use the official segment client which is async, this is what we did