[core-remote] Use mutability from RefCell<Hub> for Registry and remove FutureId

[sagudev]

Connections
Towards #10073

Description
As outlined in #10074 (comment) we replace refcell in Registery with RefCell. I am very happy with end results as there are no more guards and much only one borrow per global method.

In second commit I remove FutureId because we do not need it really anymore (it made sense when it did not hold &mut) and it's unnecessary complication.

Testing
Just refactor.

Squash or Rebase?
Rebase.

Checklist

• I self-reviewed and fully understand this PR.
• WebGPU implementations built with wgpu may be affected behaviorally.
• Validation and feature gates are in place to confine behavioral changes.
• Tests demonstrate the validation and altered logic works.
• CHANGELOG.md entries for the user-facing effects of this change are present.
• The PR is minimal, and doesn't make sense to land as multiple PRs.
• Commits are logically scoped and individually reviewable.
• The PR description has enough context to understand the motivation and solution implemented.

[teoxoy]

Good stuff!