AVRO-4332: [java] Add support for enum default values in IDL serialization

IdlUtils.writeSchema did not emit the enum default when serializing a
schema to IDL, even though IdlReader parses it. As a result, an enum
default was silently dropped when round-tripping a schema through IDL.

Append "= <default>;" after the enum body when the schema has a default,
matching the IDL grammar (RBrace defaultSymbol=enumDefault?), and add
tests covering the with-default, without-default and write-then-parse
round-trip cases.