Support `CREATE EXTERNAL TABLE dt STORED AS DELTATABLE` syntax with datafusion

[avantgardnerio]

Description

In datafusion, SQL users can use the create external table xyz stored as csv option to register CSV, JSON, and parquet data. It would be nice (essential in my company's case) to allow users to register deltalake tables in the same manner.

[MrPowers]

@avantgardnerio - this is awesome. Can't wait to have Datafusion support! I will definitely help update the docs / write blog posts when this functionality is added.

[avantgardnerio]

Thanks! Related datafusion PR if you'd like to follow along: apache/datafusion#3907 . I've definitely seen it work in Datafusion, so I don't think support is far away for that. Ballista will be some more work as we'll need to do serde on the physical plan, and re-register the object store on the executors.