Support parsing and generating OpenAPI spec for file uploads

[greyli]

• Add File field and form_and_files location
• Fix the content type of form and files input locations.
• Raise error if the user uses multiple body input locations ("files", "form", "json").
• Add Form field and form_and_files location for better form upload support.
• Rewrite the files to act like form_and_files, so that failed parsed file will be included in the returned data.

File validators will be added later in a separate PR.

The OpenAPI validation failed due to python-openapi/openapi-spec-validator#113.

• fixes Support file uploading for OAS #123

Checklist:

• Add tests that demonstrate the correct behavior of the change. Tests should fail without the change.
• Add or update relevant docs, in the docs folder and in code docstring.
• Add an entry in CHANGES.md summarizing the change and linking to the issue and your username.
• Add *Version changed* or *Version added* note in any relevant docs and docstring.
• Run pytest and tox, no tests failed.