feat(set_header): refactor and improve multiple header middleware

[seun-ja]

Motivation

Fixes #571

Offering the ability to add multiple headers at once

Solution

For better code organisation, the response.rs file, which handles the response request, is now a module and split into two files. One for setting a single header and the other for multiple, while still maintaining the current API structure.

The functions handling setting headers take a dynamic variable that implements HeaderMetadataGenerator. The newly created HeaderMetadata and tuple of (HeaderName, M) currently implemented in M can be HeaderValue (A test case covers this). This also helps with maintainability

[jlizen]

I like the intent here, but there are a few issues as written, see below.

[jlizen]

@seun-ja changes look great! Just one last bit of cleanup

[jlizen]

We should do this based on the public traits instead:

  impl<T, M> From<(HeaderName, M)> for HeaderMetadata<T>
  where
      M: MakeHeaderValue<T> + Clone + Send + Sync + 'static,
  {
      fn from((header_name, make): (HeaderName, M)) -> Self {
          HeaderMetadata::new(header_name, make)
      }
  }

[jlizen]

Great stuff! Thanks for the contribution. Will get a release out.