Fixes mypy errors in user document

[spacether]

• Fixes mypy errors in user document
• petstore: component response added with headers and no body (prevents the code from using dataclass, or if it does, no defaults may be used)
• petstore: component schema added with type object and property named key

Note:

• mypy runs on the petstore sample
• ApiResponse classes changed from dataclasses to vanilla classes because sometime body is before headers and sometimes it is flipped and sometimes they have defaults. Once python 3.10 is used, they can be changed back to dataclass and use kw_only=True.

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:

  mvn 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/python*.
  For Windows users, please run the script in Git BASH.