feat: support custom log formatters attached to handlers

[daniel-sanche]

The python logging spec allows formatters to be attached to handlers to format the output text. Right now, this is unsupported in CloudLoggingHandler and StructuredLoggingHandler, and may break some functioanlity.

We should implement and add tests around this so we can advertise this as a supported feature.

[daniel-sanche]

You should be able to set a formatter at the "setup_logging" level