Add record_multiplexer microbenchmarks

[ballard26]

This PR adds benchmarks for record_multiplexer along with the following that was needed to support that;

• A cmake-build-compatible random Protobuf message generator.
• Protobuf support for record_generator.
• A serde parquet writer that writes to a null ostream.

Backports Required

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

Release Notes

• none

[ballard26]

Is there any existing serializer for this message index format anywhere in our code-base? There is a de-serializer; get_proto_offsets in src/v/datalake/schema_registry.h. Happy to move this serializer to a more general location if there is any use for it outside of the record generator.

[andrwng]

Closest thing is

redpanda/src/v/datalake/tests/record_schema_resolver_test.cc

Lines 63 to 72 in 514f1df

	iobuf encode_pb_offsets(const std::vector<int32_t>& offsets) {
	auto cnt_bytes = vint::to_bytes(offsets.size());
	iobuf buf;
	buf.append(cnt_bytes.data(), cnt_bytes.size());
	for (auto o : offsets) {
	auto bytes = vint::to_bytes(o);
	buf.append(bytes.data(), bytes.size());
	}
	return buf;
	}

I don't have strong feelings about code placement, I think leaving it in the record generate seems reasonable

[vbotbuildovich]

ducktape was retried in https://buildkite.com/redpanda/redpanda/builds/58542#019352d8-d1a4-477d-92c3-9735aa6242dc