[Rust Server] Support header objects

Created by: richardwhiuk

Previously, object types present in Headers would fail to compile with errors of the form:

cargo build
   Compiling openapi-v3 v1.0.7 (/data/developer/swagger-codegen/samples/server/petstore/rust-server/output/openapi-v3)
error[E0277]: the trait bound `swagger::IntoHeaderValue<models::ObjectHeader>: std::convert::From<hyper::header::HeaderValue>` is not satisfied
   --> output/openapi-v3/src/client/mod.rs:697:22
    |
697 |     object_header: (*Into::<swagger::IntoHeaderValue<models::ObjectHeader>>::into(response_object_header)).clone(),
    |                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `std::convert::From<hyper::header::HeaderValue>` is not implemented for `swagger::IntoHeaderValue<models::ObjectHeader>`
    |
    = help: the following implementations were found:
              <swagger::IntoHeaderValue<chrono::DateTime<chrono::Utc>> as std::convert::From<hyper::header::HeaderValue>>
              <swagger::IntoHeaderValue<i16> as std::convert::From<hyper::header::HeaderValue>>
              <swagger::IntoHeaderValue<i32> as std::convert::From<hyper::header::HeaderValue>>
              <swagger::IntoHeaderValue<i64> as std::convert::From<hyper::header::HeaderValue>>
            and 7 others
    = note: required because of the requirements on the impl of `std::convert::Into<swagger::IntoHeaderValue<models::ObjectHeader>>` for `hyper::header::HeaderValue`
    = note: required by `std::convert::Into::into`

This work adds support for objects in headers. It was necessary to move the definition of IntoHeaderValue from the swagger crate to being part of the auto-generated code so that we could convert between IntoHeaderValue and hyper::HeaderValue for models defined by the API.

It also adds tests to ensure this continues to work.

Rust Technical Committee

• @frol
• @farcaller
• @bjgill

PR checklist

• Read the contribution guidelines.
• If contributing template-only or documentation-only changes which will change sample output, build the project before.
• Run the shell script(s) under ./bin/ (or Windows batch scripts under.\bin\windows) to update Petstore samples related to your fix. This is important, as CI jobs will verify all generator outputs of your HEAD commit, and these must match the expectations made by your contribution. You only need to run ./bin/{LANG}-petstore.sh, ./bin/openapi3/{LANG}-petstore.sh if updating the code or mustache templates for a language ({LANG}) (e.g. php, ruby, python, etc).
• File the PR against the correct branch: master, 4.3.x, 5.0.x. Default: master.
• Copy the technical committee to review the pull request if your PR is targeting a particular programming language.