Common: consider adding custom formats for JSON Schemas

[chuwy]

Until now I mistakenly thought that formats property must be equal to one of predefined formats (uuid, ipv4 etc), but I just found out that it may be a custom property. Actual implementation of this property is on the client app's (iglu-client, igluctl) behalf.

We already implemented Library interface (fge schema validator's entity responsible for adding custom properties and formats) for a self property, on top of draft v4. May be we want to add new formats to it as well? Something iglu-specific? (not supporting implementations must skip it) jsonpath, schemaver, schemaCriterion?

@alexanderdean any thoughts? Do we need it?

Also I think what lib this Library (~100 LOC javaish class) should belong to. It should be shared among at least igluctl and Iglu client, but client now uses duller approach with catching exceptions. DDL AST is probably best option.

[alexanderdean]

Interesting! Where does this Library live today?

[chuwy]

In igluctl.

[alexanderdean]

Cool, have put this into the Far-out stuff milestone for now...