[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

检查 gax-java 中现有的 google-api-client 和 google-http-client 配置，然后阅读所引用的 googleapis/java-datastore pull request 928，以了解受影响的依赖设置。确定一个更靠近 google-api-client 的合适位置，并验证 java-datastore 模块可以使用这些配置而无需 gax。

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