Move usm_ndarray into dpctl_ext.tensor

[vlad-perevezentsev]

This PR proposes to migrate the tensor interface (usm_ndarray, dlpack, flags) into dpctl_ext/tensor making dpnp independent of dpctl's tensor module.

Updates:

• Introduce dpctl_ext_capi.h
• Implement a clean CMake interface library DpctlExtCAPI to properly propagate generated headers to consumers
• Update remaining imports from dpctl.tensor to dpctl_ext.tensor
• Link all backend extensions against DpctlExtCAPI to ensure consistent access to the C-API

• Have you provided a meaningful PR description?
• Have you added a test, reproducer or referred to an issue with a reproducer?
• Have you tested your changes locally for CPU and GPU devices?
• Have you made sure that new changes do not introduce compiler warnings?
• Have you checked performance impact of proposed changes?
• Have you added documentation for your changes, if necessary?
• Have you added your changes to the changelog?

[github-actions]

View rendered docs @ https://intelpython.github.io/dpnp/pull/2807/index.html

[github-actions]

Array API standard conformance tests for dpnp=0.20.0dev3=py313h509198e_262 ran successfully.
Passed: 1355
Failed: 2
Skipped: 16