[Rust] Dynamic protobuf support

[andrijast-db]

What changes are proposed in this pull request?

Closes #372

Adds a native dynamic-protobuf path for ingesting records whose schema is known only at runtime, with no compiled prost::Message type.

New public API in the dynamic module:

• StreamBuilder::dynamic_proto(descriptor) - select the format from a runtime DescriptorProto.
• ZerobusStream::message_descriptor() / new_record() - resolve the schema (cached) and start a record.
• dynamic::DynamicRecord - build records field-by-field with a validating, chainable .set(field, value)?.
• dynamic::message_descriptor() standalone resolver, the IntoDynamicValue conversion trait, and re-exports of prost_reflect::{DynamicMessage, MessageDescriptor, Value}.

Follow-ups:

• Add ability to fetch schema from UC

How is this tested?

Manually tested with custom Rust client

[teodordelibasic-db]

This now has the same signature as compiled_proto and collapses into the same (RecordType::Proto, Some(*desc)) build arm, the DynamicProto variant currently differs only in its Debug label, and message_descriptor()/new_record() work on compiled_proto streams too since they only check for a descriptor. So the second method adds a name but no distinct behavior.

I think we should look into dynamic_proto accepting the resolved descriptor instead:

pub fn dynamic_proto(mut self, descriptor: dynamic::MessageDescriptor) -> Self

The caller resolves once up front via dynamic::message_descriptor(&proto)? (or we add a descriptor_from_uc_columns to resolved-descriptor path), and the builder pulls the raw DescriptorProto back out for the CreateStream payload with MessageDescriptor::descriptor_proto().encode_to_vec(), wire-identical to today. This makes the method genuinely different from compiled_proto, fails fast on a bad descriptor at build() instead of at the first new_record(), and lets us drop the lazy OnceLock<MessageDescriptor> on ZerobusStream (the descriptor is now supplied, not lazily rebuilt).

[andrijast-db]

Makes sense. Switched to MessageDescriptor.