Allow overriding the serializer used by the JSON renderer

[petemounce]

https://www.structlog.org/en/stable/performance.html recommends using a more performant JSON serializer than stdlib's. It would be great to allow passing that into the FormatAsCloudLogging.

[multani]

@petemounce I released the change in https://github.com/multani/structlog-gcp/releases/tag/v0.2.4 🎉

[multani]

@petemounce FYI, I released https://github.com/multani/structlog-gcp/releases/tag/v0.3.0 which provides a new builder function that doesn't configure the JSON renderer by default and focus exclusively on the GCP-specific processors.

See the "Advanced Configuration" doc, where I show an example on how to use it and how to pass an explicit JSON renderer.