[java-datastore] Relocate GoogleJsonClientFeature configurations closer to google-api-client [requires gax]

Configurations related to `google-api-client` and `google-http-client` are currently present in gax-java. Libraries like java-datastore contain modules that bring in the google-api-client dependency but not gax so they aren't able to take advantage of these configs and as a result, have to add some of these configurations on their own. Reference: https://github.com/googleapis/java-datastore/pull/928