Common: introduce the concept of an "origin" schema format to Iglu

[chuwy]

What should be considered as master copy of Iglu registry?

We have two possible options:

• Iglu registry is just original Schemas, all derived assets (like Redshift DDLs not derived from JSON Schemas) are secondary, thus can be generated automatically
• Iglu registry is set of all Schemas it contains, no matter whether they're original or derived from other.

This question is really boils down to what we expect from derivation tools like igluctl static generate. If we expect them to be ideal, them we need to stick with first option, otherwise if human-interaction is unavoidable then we need to chose second option.