[request]: Support serialization / deserialization of data structures (regression testing)

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**Tell us about your request**
Support serialization and deserialization of Textract and other data structures returned as a result of queries to AWS services

**Tell us about the problem you're trying to solve. What are you trying to do, and why is it hard?**
For regression testing, I find it useful to cache Textract results to make sure they are being processed correctly for known documents. For this use case, it is useful to save the returned JSON and be able to deserialize it to the same data structures
which are returned from live calls.

**Are you currently working around this issue?**
I currently do this in Rusoto, it would be convenient to be able to deserialize saved JSON back to a GetDocumentAnalysisOutput structure. It looks like the existing serialization code is private and designed to work with HTTP responses.

If there is already an existing mechanism for this, please let me know.

**Additional context**
The library works really well - thank you!

**Attachments**
None

Start by examining the existing private serialization code mentioned in the issue and the Rusoto approach used by the reporter. Use saved JSON and GetDocumentAnalysisOutput as the concrete regression-testing case; done means the returned data structure can be serialized and deserialized for offline processing.

Written by the indexing model from the issue text.