Provide a low-level api to write multiple non-contiguous byte-slices into a message

[bennetthardwick]

From the discussion in #1359.

It would be a great to have a way to write the data section of a message without needing to buffer the entire message in memory first.

This would be useful in cases where:

• you're streaming large messages over the network and writing to an MCAP file
• you're using a data structure where data is non-contiguous in memory (IoSlice, zenoh::Sample)
• you're memory constrained and don't want to allocate entire messages before writing

Similar to how writing attachments works - writing messages could be broken up into the following steps:

• start_message(message_header): write the len, message header and put the writer into the "writing message" state
• put_attachment_bytes(&bytes[..]): write bytes to the message
• finish_message: complete the message, verify the len, etc. is correct

There might be some challenges around writing messages when chunks are or are not enabled, but I imagine there would just be separate entries in the WriteMode enum.

[linear[bot]]

FG-11794 Provide a low-level api to write multiple non-contiguous byte-slices into a message