Support DynamoDB as a backend

[ajenkinski]

I'm opening this issue as a feature request to add support for using an AWS DynamoDB table as the cache backend. I have already implemented a BaseCache implementation that uses DynamoDB, and I'm opening this issue so I can submit a pull request, and link it to this issue.

DynamoDB is a convenient cache backend for AWS users. It's a serverless service, so there's no redis or memcache cluster to maintain. It has a builtin TTL feature to automatically delete items when they expire based on a timestamp field in the items. If a table is configured with PAY_PER_REQUEST billing mode, then you only pay for the throughput you use, which is great for low traffic services. I don't know how its performance compares to redis or memcache; the main appeal for me is the ease of setup and maintenance, and low cost for low traffic services.