Arine

Results 2 issues of Arine

For import consistency, put ResponseHeader into pingora_http crate's prelude. This enables importing with `use pingora::prelude::*` to bring ResponseHeader and RequestHeader together. Fixes https://github.com/cloudflare/pingora/issues/551.

enhancement
Accepted

## Describe the bug This is a minor bug for import consistency. When using pingora crate to follow examples, most of the required things are imported from `pingora::prelude::*` including pingora_http::RequestHeader,...

good first issue
ergonomics