SPR-9488 Add GsonHttpMessageConverter

This commit adds support to read and write JSON using the Google Gson
library. GsonHttpMessageConverter offers default Gson configuration, but
can be customized by using GsonFactoryBean. GsonFactoryBean includes
several convenience properties for configuring the internal GsonBuilder
and the resulting Gson object.

By default Gson converts byte arrays to JSON arrays instead of a Base64
encoded string. GsonBase64ByteArrayJsonTypeAdapter provides support to
read and write Base64 encoded byte arrays, and can be enabled in
GsonFactoryBean.

RestTemplate will enable GsonHttpMessageConverter only if Jackson 2 is
not found on the class path, because by default GsonHttpMessageConverter
supports the same media types as Jackson.

Issue: SPR-9488