Add metadata extractor for dbt

[ravisuhag]

Write a metadata extractor for dbt

Deliverables

• [ ] describe the metadata that can be returned in comments here.
• [ ] describe the data model that can be used for output, i.e, the metadata is in form of Table, Dashboard, User, etc.
• [ ] add unit tests
• [ ] add extractor
• [ ] add README.md in plugins/extractors/presto, defining output
• [ ] register your extractor plugins/extractors/populate.go
• [ ] add extractor the extractor list in docs/reference/extractor.md