Type of request: This is a ...

[x] bug report
[ ] feature request

Detailed description

register_stream_consumer returns ConsumerCreationTimestamp in the format '%Y-%m-%dT%H:%M:%S.%fZ', but according to the AWS CLI docs (also this)it should be a UNIX timestamp.

This breaks using register_stream_consumer with software.amazon.awssdk.services.kinesis.KinesisAsyncClient since it can't unmarshall the date in the response:
software.amazon.awssdk.core.exception.SdkClientException: Unable to parse date : 2020-08-11T05:10:06.349Z

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Expected behavior

ConsumerCreationTimestamp should be a float UNIX timestamp
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Actual behavior

ConsumerCreationTimestamp is a string formatted timestamp
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Steps to reproduce

Run any register_stream_consumer command

Command used to start LocalStack

Java testcontainers/localstack

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Client code (AWS SDK code snippet, or sequence of "awslocal" commands)

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