Validate vendor extensions

[jisantuc]

Improvement

Currently users are limited to extension validation of the few extensions we've mapped into Scala. That's fine for us since those are obviously the Best Extensions ™️ but it's conceivable that at some point in the future one of two things will be true:

• someone will want to be able to validate a private extension
• someone will want to be able to validate an extension that hasn't yet ascended to the higher plane of being of the label extension

Those are both reasonable wants!

Notes + Context

I have an idea for how this could work. It relies on:

• a startup option pointing to a json file containing objects with extension names and links accessible to the Franklin server to the schemas
• options for the services that serve items / collections to take an "extra extensions" map
• fallback to the extra extensions map if the built-in extension name checks don't find anything

[jisantuc]

Done in #664