sdk: implement AutoCloseable

close() method is invoked automatically when API proxy is
managed by the try-with-resources statement

try (Api api = new Api(URL, ...)) {
  ...
} catch (Exception e) {
  ...
}

Change-Id: I15786f0f1857c54983602de1f312b234de99bca4
Signed-off-by: Michael pasternak <mpastern@redhat.com>