datalog struct decoder

Saw this CD thread, seems like something like what's described here should be in robotpy? https://www.chiefdelphi.com/t/parsing-the-struct-definition-for-structs-in-wpilog-from-python/521542/7

Start with the linked Chief Delphi discussion to determine the intended struct-decoding behavior, then locate the relevant datalog implementation in mostrobotpy. No file, test, input format, or completion criteria is named, so define those requirements before deciding what done looks like.

Written by the indexing model from the issue text.