Multipart 3.0.0

Multipart 3.0 is here, now with Codable support! 🏞 Use multipart to parse and serializes multipart-encoded data.

Docs:
https://docs.vapor.codes/3.0/multipart/getting-started/

API Docs:
https://api.vapor.codes/multipart/latest/Multipart

Milestone:
3.0.0

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Changes since final release candidate.

⚠️ The old hacky way of using MultipartForm to parse requests is gone. Use Content just like JSON and form-urlencoded.

New:

• Codable support! Use FormDataDecoder and FormDataEncoder to convert multipart data to Swift.

Fixed:

• API audit: Lots of APIs cleaned up and/or made internal. Lots of doc blocks added, too!