protobuf: actually output map types

[rockwotj]

Maps are weird in proto3. Internally they are a repeated synthetic message type with a key and value, because we're not checking if the type is a map, we're outputting those as messages. Fix that by when we complete a message see if it's a map entry then convert it to the proper internal map type.

Backports Required

• [ ] none - not a bug fix
• [ ] none - this is a backport
• [x] none - issue does not exist in previous branches
• [ ] none - papercut/not impactful enough to backport
• [ ] v24.2.x
• [ ] v24.1.x
• [ ] v23.3.x

Release Notes

• none

[vbotbuildovich]

ducktape was retried in https://buildkite.com/redpanda/redpanda/builds/55061#019225c5-d589-41bb-b050-38737ebb1af4

ducktape was retried in https://buildkite.com/redpanda/redpanda/builds/55061#01922669-c8f3-4e8e-aa82-4342839dfd7f