Add support for completely disabling multipart request handling

[ThisIsMissEm]

As discussed on discord, by the multipart request handling always being enabled, there can be security implications. This adds a multipart.enabled option, which defaults to true, which allows disabling handling for multipart requests.

If multipart.enabled === false then a multipart request is rejected immediately with a HTTP 514 UNSUPPORTED_MEDIA_TYPE response. As mentioned in that documentation, we could also set the Accept-Post / Accept-Patch header to indicate which content-types are supported by the server, however, this would need something specific in Exception class from poppins.

🔗 Linked issue

Discussed with @thetutlage on discord today.

❓ Type of change

• 🐞 Bug fix (a non-breaking change that fixes an issue)
• 👌 Enhancement (improving an existing functionality like performance)
• ✨ New feature (a non-breaking change that adds functionality)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

This allows people using Adonis Framework to improve the security of their servers by allowing them to completely disable multipart request handling. Previously you needed a custom middleware to handle this.

📝 Checklist

• I have linked an issue or discussion. — n/a
• I have updated the documentation accordingly.

[ThisIsMissEm]

Documentation PR: adonisjs/v6-docs#191

[limwa]

Wouldn't it be better to create a custom exception class for this and expose it via an errors export, like the @adonisjs/auth package?
That way, people can use instanceof errors.... in an error handler.

However, this would require new documentation for the new exception type...

[ThisIsMissEm]

I followed the pattern already in use here: https://github.com/adonisjs/bodyparser/blob/develop/src/bodyparser_middleware.ts#L72-L89

[limwa]

Oh, I see! Thank you for clarifying!