sched: Add an initial sketch of the find_proxy_task() function

sched: Add an initial sketch of the find_proxy_task() function

Add a find_proxy_task() function which doesn't do much.

When we select a blocked task to run, we will just deactivate it
and pick again. The exception being if it has become unblocked
after find_proxy_task() was called.

This allows us to validate keeping blocked tasks on the runqueue
and later deactivating them is working ok, stressing the failure
cases for when a proxy isn't found.

Greatly simplified from patch by:
  Peter Zijlstra (Intel) <peterz@infradead.org>
  Juri Lelli <juri.lelli@redhat.com>
  Valentin Schneider <valentin.schneider@arm.com>
  Connor O'Brien <connoro@google.com>

[jstultz: Split out from larger proxy patch and simplified
 for review and testing.]
Signed-off-by: John Stultz <jstultz@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Tested-by: K Prateek Nayak <kprateek.nayak@amd.com>
Link: https://lkml.kernel.org/r/20250712033407.2383110-7-jstultz@google.com