Add C API functions back to _usmarray.pyx

[ndgrigorian]

Removal of C API functions meant that if a downstream extension were to use any getters implemented by dpnp::tensor::usm_ndarray, it would require recompilation if the ABI is changed.

C API functions prevent this

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[github-actions]

View rendered docs @ https://intelpython.github.io/dpnp/pull/2866/index.html

[coveralls]

coverage: 78.307% (-0.1%) from 78.429% — add-api-functions-to-usmarray into master

[github-actions]

Array API standard conformance tests for dpnp=0.20.0dev6=py313h509198e_18 ran successfully.
Passed: 1355
Failed: 5
Skipped: 16