RavenDB: Get IDocumentStore from DI if available

[georgiosd]

What would you like to be added:

For RavenDB:

• Look up IDocumentStore in DI before constructing your own
• Since construction also requires options, provide an overload without options

Why is this needed:

Document store initialization is costly and providing options for apps that already configure RavenDB on their own is cumbersome.