TypeDecoder handling of [String: Any] for Swagger

[djones6]

A user may wish to assign an [String: Any] type on a struct for the purposes of passing through opaque JSON whose structure is unknown. We currently do not support type decoding of [String: Any].

When it comes to encoding/decoding such structures, this may be relevant: https://medium.com/@scotthoyt/swift-4-bridging-codable-json-and-string-any-1b76b9df2b2e