Allow configuration of log group and stream prefix for CloudwatchAgentExtension

[SamStephens]

I've used the logGroup parameter of Container to consumer awslogs for a Service. Within that logGroup, the logStreams are named in the format /[ServiceConstructId]/app/[FargateTaskId].

I would like my Cloudwatch Agent logs to go to the same log group, and possibly the same log stream. However CloudwatchAgentExtension implicitly creates a log group, and provides no configurability.