Switch from Reqwest client to Hyper client

[bnusunny]

Lambda Web Adapter use Reqwest client to send http requests, which works great. And AWS Lambda Rust Runtime lambda-http crate added support for http_body::Body in v0.6.0, see PR#491.

By switching from Reqwest client to Hyper client, we can use that feature from lambda-http crate and remove convert_body() function here. Reduce the amount of code in the adapter.

[bnusunny]

Close this one because we built a tower service and cannot leverage lambda-http PR#491.