feat: Add support for custom JSON encoders

[thiromi]

Add the ability to provide custom JSONEncoder classes to StructuredLogHandler, so the log caller doesn't have to convert to primitive types every time a log record is created

Fixes #656 🦕

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[daniel-sanche]

Thanks for putting this together. My only hesitation is that the CloudLoggingHandler also doesn't support logging unserializable dictionary types, so it might make more sense to implement this in a more universal place. It would be ideal if the handlers can be as similar as possible.

That said, I'm not seeing a clear solution for CloudLoggingHandler, since it uses a different method of parsing JSON, and never serializes into strings like the StructuredLogHandler. It's not as simple as passing in a single argument like this.

I opened a feature request to look into this in the future, but I don't think it needs to block merging this