Make location field non-required

### 🤔 What's the problem you're trying to solve?

At https://github.com/elchupanebrej/pytest-bdd-ng there is a possibility to store features in formats different from .gherkin or .md files (yaml, ndjson, toml, etc.). Third-party parsers of these formats do not always give information about consumed tokens, so it's not possible to put correct values to the Location field

### ✨ What's your proposed solution?

- Make the Location field non-required, so it won't be filled if the parser doesn't grant needed information.
- Add the possibility to add custom mime types into MediaType schema

Start by locating the schema definitions for the Location field and MediaType schema in this repository. Check how optional location values and custom MIME types are represented, then verify the schema changes across the supported message implementations; done means parsers without token locations can omit the field and callers can register custom MIME types.

Written by the indexing model from the issue text.