This commit adds a new smoke test for testing client as a end Java user.
It starts a cluster in `pre-integration-test` phase, then execute the client operations defined as JUnit tests within `integration-test` phase and then stop the external cluster in `post-integration-test` phase.
You can also run test classes from your IDE.
* Start an external node on your machine with `bin/elasticsearch` (note that you can test Java API regressions if you run an older or newer node version)
* Run the JUnit test. By default, it will run tests on `localhost:9300` but you can change this setting using system property `tests.cluster`. It also expects the default `cluster.name` (`elasticsearch`).
This commit also starts adding [snippets as defined by Maven](https://maven.apache.org/guides/mini/guide-snippet-macro.html) to help keeping automatically synchronized the Java reference guide with the current code.
Our documentation builder tool does not support snippets though but we will most likely support it at some point.
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