Add support for MIME type definitions on document types

[freakboy3742]

What is the problem or limitation you are having?

The current specification for document types lets you define an extension, description and icon for a file type - but doesn't let you define a MIME type. Some platforms (e.g., Windows) require a MIME type definition for file association purposes.

Describe the solution you'd like

It should be possible to define a mime type for a document type:

[tool.briefcase.app.helloworld.document_type.foobar]
description = "A foo bar document"
extension = "fbr"
icon = "icons/foobar"
url = "https://example.com"
mime_type = "text/plain"

Describe alternatives you've considered

Status quo - an autogenerated mime type based on the document type ID and application name.

Additional context

We could do some validation of the mime type; there is a set of known registries that should be the only valid starting text for a mime type (e.g., application/..., text/...).

Once these this is formalised, we should ensure all app templates use the mime type as appropriate.

Originally came from the discussion on #1706, and the implementation in beeware/briefcase-windows-app-template#62