[Datasets] Support MongoDB as a datasource for Dataset

[jianoaix]

This is a feature request to add MongoDB as a datasource to read and create Dataset. This will be supported by implementing a custom DataSource.

Use cases:

• MongoDB is a popular non-SQL database (e.g. ranked 5th on https://db-engines.com/en/ranking, and all 4 DBs above are SQL databases), so it's useful to have support from Datasets.
• We actually have customers using MongoDB in production. Without this integration, the customers have to build their batch prediction application on top of Ray Core APIs, which is much more demanding -- it'll take more effort, long time and then produce an application that's more prone to issues due to larger surface area of lower-level APIs.