feat(screenshot): allow downstream Xbox SDK to supply capture implementation

[tustanivsky]

This PR allows the downstream sentry-xbox extension to provide its own implementation of sentry__screenshot_capture by skipping the bundled Windows screenshot module when targeting Xbox.

Related items:

• https://github.com/getsentry/sentry-xbox/pull/135

#skip-changelog

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[tustanivsky]

Screenshot capturing on Xbox requires changes from this PR in the upstream repo, but those changes also depend on the feature existing in the downstream repo for the console CI checks to pass.

To work around this chicken-and-egg issue, we can merge this PR while temporarily ignoring the console/xbox failure and tag sentry-xbox with the corresponding commit SHA in sentry-native.cmake.

After getsentry/sentry-xbox#135 is merged, the console checks in the upstream repo should start passing again.

The only thing to keep in mind is that until the next release, sentry-xbox will reference sentry-native's main branch instead of the 0.14.0 release.

cc @jpnurmi