JsonFormatter datetime format does not include milliseconds

As defined here: https://github.com/aws/aws-lambda-python-runtime-interface-client/blob/b45577eeda88c90485811b05a8c7b66e5ed39fe1/awslambdaric/lambda_runtime_log_utils.py#L10

This is an issue for log records separated by less than a second and also when they're mixed together with records that are millisecond-accurate (such as for Lambda lifetime phases, e.g. `{"time":"2024-06-19T23:33:05.068Z","type":"platform.initStart", ...}`)