MCAP Attachment API for C++ library

[kevswims]

With the C++ MCAP API the only way to query and read attachments involves using the TypedRecordReader with an onAttachment to read through the entire file. For large MCAP files this is not particularly efficient.

An API that allows access to the summary information to access the attachment indexes would be a decent approach. Even better would be a higher level API that allows easy querying and reading of attachments.