Turn (Jackson|Gson)AutoConfiguration into true auto-configuration

Previously JacksonAutoConfiguration and GsonAutoConfiguration were
not actually auto-configuration classes. They were only processed
due to being imported by HttpMessageConvertersAutoConfiguration.
In addition to being misleadingly named, this meant that they could
not be included or excluded individually and were also tightly coupled
to HTTP message conversion.

This commit updates spring.factories to make both
JacksonAutoConfiguration and GsonAutoConfiguration actual
auto-configuration classes. As a result, they can now be enabled or
disabled individually and are no longer coupled to HTTP message
conversion.

Closes gh-1562