[haskell][server] Include response headers in the API type

[7omb]

In servant response headers are part of the type of the API. If headers are included in the response object of the OpenAPI spec they are now added to the Haskell type. If no headers are given the type of the Haskell API is unchanged.

Since headers in the OpenAPI spec is a map multiple Set-Cookie headers are currently not possible. If someone uses the workaround with null bytes, the null bytes are removed and each header is headed to the list.

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[wing328]

@7omb thanks for the PR.

This is not a breaking change (non-backward compatible), right?

cc @frol (2017/07) @farcaller (2017/08) @richardwhiuk (2019/07) @paladinzh (2020/05)

[7omb]

Changes to an endpoint would only be necessary, if the spec defines response headers. For the servant generator response headers were only for documentation purposes before. I did not check how often other generators are sensitive to response headers. If one uses the e.g. servant generator together with a client generator that is sensitive to response headers, changes would be required.

Edit:
An interesting point is that it is currently not possible to use servants addHeader Funktion since it required the type of the header in the generated API.

[wing328]

Tests passed locally:

openapi-petstore> configure (lib)
Configuring openapi-petstore-0.1.0.0...
openapi-petstore> build (lib)
Preprocessing library for openapi-petstore-0.1.0.0..
Building library for openapi-petstore-0.1.0.0..
[1 of 2] Compiling OpenAPIPetstore.Types
[2 of 2] Compiling OpenAPIPetstore.API
openapi-petstore> copy/register
Installing library in /Users/williamcheng/Code/openapi-generator/samples/server/petstore/haskell-servant/.stack-work/install/x86_64-osx/d7eaccd255e546d469fd16f5af3892ff7caa92ed8e070d4c3bfe612e97b830c4/9.0.2/lib/x86_64-osx-ghc-9.0.2/openapi-petstore-0.1.0.0-4YkDtr09w237nRoGp23ALC
Registering library for openapi-petstore-0.1.0.0..
[INFO] ------------------------------------------------------------------------
[INFO] BUILD SUCCESS
[INFO] ------------------------------------------------------------------------
[INFO] Total time:  56.690 s
[INFO] Finished at: 2022-12-06T23:47:57+08:00
[INFO] ------------------------------------------------------------------------

Let's give it a try.

Thanks for the PR.