Maps with `fixed` keys can't be serialized

[Dogacel]

Generated map fields with fixedXX and sfixedXX keys can't be serialized.

We should follow up with `kotlinx.serializationz team. I already made some attempts:

• Protobuf repeated uint, fixed and enum serialization not working Kotlin/kotlinx.serialization#2401
• Add protobuf conformance tests v1 Kotlin/kotlinx.serialization#2404