Clearly establish message lifetimes & copies

[jameskuszmaul-brt]

When poking at trying to understand performance of studio for displaying large MCAP files, I observed that a substantial percentage of time was spent doing memcpy's in the mcap code (see foxglove/mcap#405). Per discussion in that issue, it sounds like studio does currently have a certain set of expectations around memory lifetime/consistency when reading MCAP files. In order to facilitate writing performant code for custom extensions, new serialization formats, etc., studio and the typescript MCAP support libraries should provide a well-documented set of interfaces to facilitate writing effective code.