Use `UnionBuilder` in reflection-based parsing of MCAP protobuf messages

[grtlr]

Describe the bug

The reflection-based parsers are not properly parsing protobuf oneof, because of:

• https://github.com/apache/arrow-rs/issues/8033

Expected behavior

Oneof should work out of the box.

Additional context

We need to wait until we can have Arrow 56.1 as a dependency.

[pweids]

just noticed we had Arrow bumpted to 56.1 in teh-cmc's update. Is this unblocked then?