Add ability to set SK and namespace

[chekalsky]

I think it would be helpful to have an ability to set SortKey for the table for projects made with single-table design principles.

In this case having prefix (e.g. RIKIDOUCACHE# or a custom one) would be a super helpful too (RikudouSage/DynamoDbCachePsr6Bundle#6).

Single-table means structure like this:

PK	SK	value	ttl
RIKIDOUCACHE#key1	RIKIDOUCACHE#key1	value1	0
RIKIDOUCACHE#key2	RIKIDOUCACHE#key2	value2	0
APP#user1	APP#user1	username1

If this case should be handled by Decorator from RikudouSage/DynamoDbCachePsr6Bundle#7 then it would be helpful to have documented somewhere.

[RikudouSage]

The prefix is added, I will have to think a little more about how to implement the sort key.