When image digest is included in the container definition image, validation fails

[this](https://github.com/awslabs/aws-embedded-metrics-java/blob/472805d2e391a461fae8db219b9ae35fec823fcf/src/main/java/software/amazon/cloudwatchlogs/emf/environment/AgentBasedEnvironment.java#L58) sanitizes the colon but excludes the `@`, so an image of the form `some.domain.example/image:version@SHA256:digest` is sanitized to `image-version@sha256-digest`, which causes the agent to reject the log group name with `InvalidParameterException: 1 validation error detected: Value 'image-version@sha256-digest-metrics' at 'logGroupName' failed to satisfy constraint: Member must satisfy regular expression pattern: [\\.\\-_/#A-Za-z0-9]+",`