Sentry Structured Logging for Unity

[AbhiPrasad]

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https://develop.sentry.dev/sdk/telemetry/logs/

Sentry is adding support for structured logging. Let's add it to the Unity SDK!

1. Define the logs protocol and log envelope item in the SDK
2. Add the Public API (SDK options and methods) as per the docs
3. Make sure the the SDK follows the documented behavior
4. Attach default attributes to the SDK as per docs
5. Instrument popular logging libraries to send logs to Sentry. SDK maintainers can best decide what they should support.
6. Add docs for the SDK.