Introduce Named Schema Formatters

Similar to AVRO-3666, which introduces multiple schema parsers, I propose to introduce multiple, named, schema formatters.

Names can be of the form <name>[/<variant>], there the variant part is optional.

Initially, the list would be:

• json -> alias for json/pretty
• json/pretty > pretty{}printed JSON; replaces Schema.toString(true)
• json/inline -> single-line JSON; replaces Schema.toString(false)
• canonical -> Parsing Canonical Form (as per spec)

Then, after merging AVRO-3404, we can also add:

• idl -> to write schemata in IDL format, as requested in AVRO-1757