[air/tune] Internal resource management 2 - Ray Tune to use new Ray AIR resource manager

[krfricke]

Why are these changes needed?

Includes/depends on #30777

TLDR: This PR refactors Ray Tune's resource management to use a central AIR resource management package instead of the tightly coupled PlacementGroupManager.

Ray Tune's resource management currently uses a tightly coupled placement group manager. This leads to a number of shortcomings:

• The tight coupling on the manager side (e.g. PG manager keeps track of trials) prevents re-usability
• The tight coupling on the trial executor side prevents using different resource management strategies (e.g. shared or budget-based)
• It's hard to test independently. Tests for the resource management require a simulated tune setup.

To improve stability, extensibility, and maintainability, this PR moves the resource management logic into a central ray.air.execution.resources subpackage. The resource management has a simple API that works with ResourceRequests and AllocatedResources to manage requested and assigned resources, respectively. The actual resource management can then be anything - per default it is a placement group based manager, but this PR also introduces a PoC budget-based manager that can be plugged in.

The PR does not substantially change existing tests, so we can be certain that the new resource model is a fully compatible replacement for the old placement group manager.

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• I've signed off every commit(by using the -s flag, i.e., git commit -s) in this PR.
• I've run scripts/format.sh to lint the changes in this PR.
• I've included any doc changes needed for https://docs.ray.io/en/master/.
• I've made sure the tests are passing. Note that there might be a few flaky tests, see the recent failures at https://flakey-tests.ray.io/
• Testing Strategy
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  • This PR is not tested :(