feat: getter for message classes

[magi-2]

Changelog

Improves the reader API for accessing protobuf messages which are generated from the Schema objects within the DecoderFactory.

Docs

None

Description

I am using protarrow and found myself having to search for the message class objects in the DecoderFactory for the example seen below. Note that, in my current project set up, I do not have the proto message classes as a submodule / dependency. Given that mcap stores the schemas themselves, that this feature adds a lot of value so that anyone can read the logs without having to include the pb messages in their project.

import protarrow

my_protos = [
    MyProto(name="foo", id=1, values=[1, 2, 4]),
    MyProto(name="bar", id=2, values=[3, 4, 5]),
]

table = protarrow.messages_to_table(my_protos, MyProto)

The only way to access these currently is by accessing 'private' class members.

msg = reader._decoder_factories[0]._types[schema.id]
table = protarrow.messages_to_table(my_protos, msg)

The change here is a quick fix but I imagine that this can be made more ergonomic with changes to the reader class itself (python/mcap-protobuf-support/mcap_protobuf/reader.py)

I did no tests as this is very straight forward. This PR does not have to be merged though I think it's a perfect example as to how this API can be improved.

Before	After
msg_before = reader._decoder_factories[0]._types[schema.id]	msg_after = reader._decoder_factories[0].pb_message_from_schema(schema)

If this feature is of interest, let me know and we can discuss what methods could be of interest. It would be nice if the reader itself offered this API. That being said, for the reader base-class I am not sure if similar changes make sense.