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

Created by: 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.

PR checklist

• Read the contribution guidelines.
• Pull Request title clearly describes the work in the pull request and Pull Request description provides details about how to validate the work. Missing information here may result in delayed response from the community.
• Run the following to build the project and update samples:

  ./mvnw clean package 
  ./bin/generate-samples.sh
  ./bin/utils/export_docs_generators.sh

  Commit all changed files. This is important, as CI jobs will verify all generator outputs of your HEAD commit as it would merge with master. These must match the expectations made by your contribution. You may regenerate an individual generator by passing the relevant config(s) as an argument to the script, for example ./bin/generate-samples.sh bin/configs/java*. For Windows users, please run the script in Git BASH.
• File the PR against the correct branch: master (6.1.0) (minor release - breaking changes with fallbacks), 7.0.x (breaking changes without fallbacks)
• If your PR is targeting a particular programming language, @mention the technical committee members, so they are more likely to review the pull request.