[PA] Move Metadata outside of GigaCage (6/7)

Provide `root` or `metadata offset` for the methods to obtain metadata,
e.g. SlotSpanMetadata::FromObject(), FromAddr(), and ToSlotStart(), and
remove default parameter: `root = nullptr` from the methods.

Remove unused `root` from SlotSpanMetadata::SetFreelistHead(), and
PartitionBucket::SetNewActiveSlotSpan(),

Design Doc:
https://docs.google.com/document/d/1Mn-qRAWuDhS_gRL_OG2TYL-EW5D8yljaCyM40S0ocVc/edit?tab=t.0

Bug: crbug.com/40238514
Change-Id: Ie8857e47fe55b53ff7c3e0f64300fed26cbf06f7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6758538
Commit-Queue: Takashi Sakamoto <tasak@google.com>
Reviewed-by: Yuki Shiino <yukishiino@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1492598}
NOKEYCHECK=True
GitOrigin-RevId: 000352b61a6701c4014ec823d0099bb3d1be8953