Ability to use `input_protobuf_encoded` from the WebUI

[Alia5]

Discussions are sadly not open, so here's an issue instead.
Note that I did not look at the code, as I've said, I would have opened a discussion instead, if that was possible

I'd like to see / propose a way that would make it relatively easy to use input_protobuf_encoded from the WebUI.

The webpage gives a pretty clear warning about fields generated from protobuf and the error-prone to input_json conversion.

⚠️ This method includes fields generated from protobufs, input_json was generated, which is very error prone. You may be able to get it to work by fiddling with the JSON manually. Please do not report issues when this doesn't work.

For a WIP project of mine I've build / am working on an ergonomic to use solution and have done it with relatively little effort