Allow editing protobuf message content

[pimterry]

Since #80 was resolved, you can now view protobuf content, but it would also be useful to be able to edit it too. There's two main use cases for this:

• Editing body content at a breakpoint
• Defining response body content in rules, or request body content when sending requests

A schema is required to do this, but we may be able to get by autogenerating the schema internally (#560) and generating content for that. We will definitely need some way to mark content as 'invalid' though during editing though (which we don't really have for other editing cases, which are effectively just highlighting & validation, not encoding) which is a bit tricky from a UX POV.

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