refactor: centralizing shared-allocation accounting for Arc DFHeapSize impls

[saadtajwar]

Which issue does this PR close?

• Closes Centralize shared-allocation accounting for Arc DFHeapSize implementations #22867

Rationale for this change

DFHeapSize depends on a critical invariant: shared heap allocations must be counted once per traversal context. If Arc implementations drift, memory accounting becomes inconsistent and hard to reason about.

Consolidating the one-time-accounting logic improves:

Correctness durability (single source of truth for Arc dedup behavior)
Maintainability (less repeated logic)
Reviewability (future changes touch one helper)

What changes are included in this PR?

Helper functions for Arc allocation identity and deduplication, namely for providing pointer extraction & DFHeapSizeCtx set membership checks, with the DFHeapSize implementations on Arc backed types using the new helpers

Are these changes tested?

Yes, the below are passing:

cargo test -p datafusion-common heap_size --lib
cargo test -p datafusion-common --doc heap_size

Are there any user-facing changes?

No

[saadtajwar]

cc @kosiew - thanks in advance for any feedback! :)

[mkleen]

@saadtajwar Thanks for working on this! I left a comment.

[mkleen]

We could add this function directly to the DFHeapSizeCtx:

impl DFHeapSizeCtx {
    fn count_allocation_once(mut self, ptr: usize) -> bool {
        self.seen.insert(ptr)
    }
}