Add support for multipart/form-data to AsyncAPI

[miguelvr]

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Description

AsyncAPI currently only supports strings, JSON, and binary payloads. Multipart/form-data is a very common payload type and could be particularly useful for this API type.

Motivation

This would improve the UX and spare the user of having to parse the binary payload.

Additional context

• This library could be useful https://github.com/defnull/multipart
• werkzeug (flask) also has a utility function to parse multipart/form-data payloads (https://github.com/pallets/werkzeug/blob/master/tests/test_formparser.py)