Add DynamoDB integration with PHPSessions

[Nyholm]

This will fix #527

The PR is currently in draft. We need to update all instances of $this->connection.

@Nemo64 feel free to continue this work if you want to.

[Nemo64]

Ohh wow. I will definitely look into this. Thanks.

So you are ok with copying the official implementation and just adjusting it, right?

[Nyholm]

Yes. I think the end result will not be super similar. I also think that there are not many ways one could implement this specific interface using DynamoDB.

I added a reference in the readme

[Nemo64]

I started porting the rest of the SessionHandler but I must admit that I won't have the time to properly implement, test and document it the coming weeks.

Here is what I have so far: https://github.com/Nemo64/aws/tree/feature/sessions/src/Integration/Aws/DynamoDbSession/src

If someone else wants to pick it up I'm for that or it'll need to wait a bit.

I have 2 notes so far:

• I omitted the entire garbage collection logic from the original. It isn't needed in modern DynamoDB since there now is a TTL feature that should be preferred in any case since it is free and requires no further implementation other than configuring it one time.
• I'm a bit concerned about the DynamoDB implementation in this library. Wrapping every value into an AttributeValue object is probably a performance problem and it also makes it a lot harder to read values as you normally can just use current($item) to read the value and now you need to know the type to call the right getter. Not a big deal for the session handler but may be for someone seriously using the DynamoDB as main data storage.