rust: support writing un-chunked MCAP files

[james-rms]

Right now, the rust writer only supports writing chunked MCAP files. It's really useful to be able to write un-chunked MCAP files.

[neilisaac]

I assume the main advantage of non-chunked writing is to implement zero-copy message writing? Is that an implicit requirement?

[james-rms]

Writing without chunks is a good use-case for embedded devices that don't want to maintain a chunk-sized buffer of message data at all times. Zero-copy message writing is also possible with un-chunked MCAP writing, and that would be a really nice feature, but not a requirement.