[java][client][restassured] add getAllOperations()

Created by: jmini

The client based on rest-assured is mainly used for testing.

We have the case, where we would like to perform a call for each of the operations available on an API during a test. This PR provide a way to solve this.

It adds a way to get all the Oper objects available on an API: getAllOperations(). Then execute(Function) (which is the common denominator between operations) can be called on the returned object in the list.

Example usage:

LoremApi api = initApi();
api.getAllOperations().forEach(o -> {
    o.execute(r -> {
        Assertions.assertThat(r.statusCode()).isEqualTo(403);
        Assertions.assertThat(r.statusLine()).isEqualTo("Forbidden");
        return r.andReturn();
    });
});

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Java: @bbdouglas (2017/07) @sreeshas (2017/08) @jfiala (2017/08) @lukoyanov (2017/09) @cbornet (2017/09) @jeff9finger (2018/01) @karismann (2019/03) @Zomzog (2019/04) @lwlee2608 (2019/10)

Rest-assured: @viclovsky