Unify the logging API

[nirinchev]

Description

This introduces a new Logger class that will be used (eventually) for all Realm-originating messages. Currently it's used for messages from Sync and those from managed code. We'll need to refactor OS to ensure that Core-originating messages also use the SDK-provided logger.

The motivation of this entire thing is twofold:

1. There are some platforms where stdout/stderr output is swallowed - such as the dotnet cli: Expose an API for SDKs to set a custom logger realm-core#4485
2. Unity doesn't have a way to output Console.WriteLine messages and instead uses its own Debug.Log mechanism: Use Debug.Log as default logger for Unity projects #2277

TODO

• Changelog entry
• Tests (if applicable)
• Filter messages based on the log level

[LaPeste]

It's overall a good idea to have a unified logger whose implementation is supplied by the SDK. It'll make life easier for everyone developing on the SDKs.