Avoid superfluous copy of dpctl_capi struct

[oleksandr-pavlyk]

This PR eliminates superfluous copying of dpctl_capi instance, even if it is later eliminated by the compiler.

The copying would necessary in static dpctl_capi api = lookup() line, since lookup returns dpctl_capi instance by value.
The change is to make it return a reference.

Since dpctl_capi is a singleton, this change only eliminates a single copy, so consider it a technical debt cleanup change.

The PR also adds a default move assignment operator for dpctl_capi.

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[github-actions]

Deleted rendered PR docs from intelpython.github.com/dpctl, latest should be updated shortly. 🤞

[coveralls]

coverage: 87.832%. remained the same
when pulling 1b19fb7 on eliminated-uncessary-copies-in-dpctl-capi
into 1fac073 on master.

[github-actions]

Array API standard conformance tests for dpctl=0.17.0dev0=py310h15de555_130 ran successfully.
Passed: 906
Failed: 0
Skipped: 94

[ndgrigorian]

LGTM!