Implement PGXN Meta JSON Schema

[theory]

Implement full validation of PGXN Meta v1 and v2 in JSON Schema so that it can be used both by our tools and any other uses by the community.

[theory]

Done in:

• Implement JSON Schema for PGXN Meta Spec v1 meta#3
• Fully test schema and fix issues found meta#4
• Implement v2 Spec JSON Schema meta#6