Session: Add support for Amazon DynamoDB as session storage backend

[jeastham1993]

Feature Request

Add an additional SessionStore implementation to use Amazon DynamoDB as the session storage backend, as an alternative to Redis.

Add an implementation that uses DynamoDB instead of Redis, allowing configuration of the table, key and TTL names. I can contribute a PR for this implementation.

Context

Building a serverless application on AWS using Actix, and want to avoid using any AWS services that rely on a VPC. Currently I need to run an Elasticache cluster to get access to Redis.

[robjtede]

Since DynamoDB allowed TTL in records it is easily satisfies the requrements for the SessionStore trait. I'd be happy to see support for it included in actix-session.

[jeastham1993]

Great, thanks @robjtede. I'll aim to get a PR raised this week.