[Structured logging] Types

[lcian]

Description

Implement types required for structured logging.

[AbhiPrasad]

I pushed up a branch of for this in abhi-logs-protocol: https://github.com/getsentry/sentry-rust/tree/abhi-logs-protocol

Was hoping to open a PR, but couldn't get the time to finish it up. Hopefully this helps as a starting point when looking at this!

[lcian]

Thank you @AbhiPrasad !