Add support for MongoClient APIs

[cmelchior]

Closes #972

This PR add support the MongoClient APIs found in Realm Java and other SDKs.

Note, that the implementation should not use the C-API as we want to prepare the code to be split from Device Sync.

TODO

• Figure out if we should support watch or not. .NET does not and it was removed from JS Web. I assume we can leave this out for the initial release of this.
• Move tests from Realm Java.
• Right now we are duplicating methods to make it possible to override the return class. Figure out if there is a better way of doing this.
  • We already have a MongoCollection.withDocumentClass(), maybe just require that people use that instead of duplicating methods?
• Figure out if we should keep using FindIterable and friends, move to FindFlow (similar to the Kotlin Driver), or both.
• Review docs. Right now I just ported the Java doc more or less as is.