JsonFormatter datetime format does not include milliseconds

[dtcaciuc-ggs]

As defined here:

aws-lambda-python-runtime-interface-client/awslambdaric/lambda_runtime_log_utils.py

Line 10 in b45577e

_DATETIME_FORMAT = "%Y-%m-%dT%H:%M:%SZ"

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", ...})