[jaxrs-spec] Improve support for sub-resource locators in jaxrs-spec generartors

[shamus13]

Adds a public string constant by the name of PATH to generated api classes / interfaces with the content of the class level Path annotation, so the path can easily be used in sub-resource locators etc. #15052 (comment)

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@bbdouglas @sreeshas @jfiala @lukoyanov @cbornet @jeff9finger @karismann @Zomzog @lwlee2608

[shamus13]

Implements suggested solution in #15052

[shamus13]

@wing328

[wing328]

can you please resolve the merge conflicts when you've time?