[API] Cloud SDK: Support data loading from MongoDB and DynamoDB

## About
Hi there. We'd like to add support for data loading from MongoDB and DynamoDB through **CrateDB Cloud ETL** to CrateDB Toolkit's I/O subsystem, effectively its `ctk load` interface, like it works for file-based resources already.

In the patch referenced below, we can spot a `import_jobs_create_from_dynamodb` function has been added to accompany DynamoDB, and from the patch I can also easily learn about relevant job attributes. Thanks!

@juanpardo: In this spirit, may I also ask you to point out the relevant spot where I can educate myself about corresponding MongoDB semantics and properties?

## References
- https://cratedb-toolkit.readthedocs.io/cluster/
- https://cratedb-toolkit.readthedocs.io/io/
- https://github.com/crate/croud/pull/597