Add metadata extractor for druid

[ravisuhag]

Deliverables

• add unit tests
• add extractor
• add README.md in plugins/extractors/druid, defining output
• register your extractor plugins/extractors/populate.go
• add extractor the extractor list in docs/reference/extractor.md

Output must contain a Table

Explore the Table Data Model and add as many features as possible.

Table

Field	Sample Value
urn	table.urn
name	my_table
source	druid
description	table description