abstract ContainerEngineHandler into StructuredLogHandler for serverless environments

[daniel-sanche]

We currently have the ContainerEngineHandler that writes formatted logs to standard output, which are then parsed and uploaded to stackdriver by fluentd

Serverless environments have issues where logs aren't always properly flushed, so they could benefit by having the same handlers.

WE should abstract ContainerEngineHandler into StructuredLogHandler, to work for other environments

[daniel-sanche]

On second thought, we should just remove ContainerEngineHandler in v3.0.0. until then, we should leave it as-is for consistency