Add multiple headers in SetRequestHeaderLayer

[seun-ja]

• I have looked for existing issues (including closed) about this

Feature Request

One should be able to set multiple headers with SetRequestHeaderLayer.

Motivation

It's inspired by the #571 (closed by #672), which only makes it possible to add multiple headers for Responses via SetResponseHeaderLayer.

Proposal

Would pretty much be the request version of what is done with the response, still maintain backward compatibility

Alternatives

Use third-party crates like tower-default-headers or write your own middleware, which involves repetitive layers.