[aws-for-fluent-bit] default_log_group_name & default_log_stream_name

**Is your feature request related to a problem?**

We noticed logs with `kubernetes['pod_name']: tag name not found` therefore we are looking for a fallback. We believe fluent bit supports a fallback here https://github.com/aws/amazon-cloudwatch-logs-for-fluent-bit/blob/mainline/cloudwatch/cloudwatch.go#L119

**Is your feature request related to a specific Helm chart, if yes mention name of the chart?**
aws-for-fluent-bit

**Describe the solution you'd like**
Support for fallback

The request concerns the aws-for-fluent-bit chart and points to cloudwatch/cloudwatch.go in amazon-cloudwatch-logs-for-fluent-bit. Read that fallback implementation first, then inspect the chart's templates and values to determine where the log group and stream names are configured. Done means the chart supports the requested fallback and its behavior is covered by appropriate chart validation or tests.

Written by the indexing model from the issue text.