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

[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();
    });
});

PR checklist

• Read the contribution guidelines.
• If contributing template-only or documentation-only changes which will change sample output, build the project before.
• Run the shell script(s) under ./bin/ (or Windows batch scripts under.\bin\windows) to update Petstore samples related to your fix. This is important, as CI jobs will verify all generator outputs of your HEAD commit, and these must match the expectations made by your contribution. You only need to run ./bin/{LANG}-petstore.sh, ./bin/openapi3/{LANG}-petstore.sh if updating the code or mustache templates for a language ({LANG}) (e.g. php, ruby, python, etc).
• File the PR against the correct branch: master, 4.3.x, 5.0.x. Default: master.
• Copy the technical committee to review the pull request if your PR is targeting a particular programming language.

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

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[wing328]

LGTM