Adding DynamicConverter support for enums

Adding DynamicConverter support for enums

Summary:DynamicConverter doesn't currently support reading enums.
This diff addresses that by adding an enum specialization that uses integers as its underlying implementation.
A follow-up is to handle both integers and strings, but that would require a way to parse strings into enums like Thrift static reflection provides (see https://github.com/facebook/fbthrift/blob/d17b072daddbd318f9afaa5562100eaba0f890e3/thrift/lib/cpp2/fatal/folly_dynamic-inl.h#L88,L92.

Reviewed By: yfeldblum

Differential Revision: D3002569

fb-gh-sync-id: 27bf4a9d5a7844762f5311e2c777606a0e7753f0
shipit-source-id: 27bf4a9d5a7844762f5311e2c777606a0e7753f0