Avro IDL Support

[L-Lavigne]

Closes #181.

Summary of changes:

• Avro IDL support (in JSON .avpr format) for all message types and unary RPC types (no streaming RPCs currently, due to Avro limitations). The idlGen plugin generates both Protobuf and Avro IDLs.
• New @outputPackage annotation.
  • In Avro IDL, its value is used for the namespace attribute (default to "" representing the root package).
  • In Protobuf IDL, its value becomes a package declaration which will then be used by parsers in place of the java_annotation option if absent or not handled by the parsers's language.
• New @outputName annotation. If not defined, the idlGen plugin will use the source filename (without extensions) instead.
  • In Avro IDL, its value is used for the protocol attribute.
  • In Protobuf IDL, its value is currently unused.
• Handling of Option parameters:
  • In Avro IDL, they become union types with null.
  • In Protobuf IDL, the Option is stripped out and the inner type is used (this fixes a possible regression in a previous PR).
• Handling of the Empty type:
  • In Avro IDL, the type gets converted to null, or an empty arguments list if used as a request argument.
  • In Protobuf IDL, the generator now uses the fully-qualified name for the Empty type and adds the relevant import if present, to satisfy more rigorous parsers.
• Breaking changes to the @option annotation: the value can be of any type (not just String) and the quoted attribute is removed; idlGen will simply preserve quotes present or not in the Scala source file.
• All top-level annotations (@option, @outputPackage and @outputName) support unnamed parameters (ex.: @option("java_multiple_files", true) will now work). Internally, the ScalaMetaUtil class was updated to enable this.
• Unit tests have been updated to cover more cases (in addition to the new features, we check for multi-field messages, List and Option types, and Fs2 serialization).
• Updated README.md examples with Avro examples, new @option syntax and various fixes.

[juanpedromoreno]

Thanks so much for this great contribution! LGTM