[PA] Split PartitionPageMetadata into read-only one and writable one.

This is a part of ShadowMetadata CL (3/5). The entire CL is
crrev.com/c/5570783.

ReadOnlyPartitionPageMetadata is used for PartitionPageMetadata
inside the giga cage, and WritablePartitionPageMetadata is used
for PartitionPageMetadata inside the shadow pool.

The design doc of shadow metadata is
https://docs.google.com/document/d/1LvMI_c1VuT4sWUqgKOdJlrZv0UWP5e3PLJ_oOlJ4l2k

Bug: 40238514
Change-Id: If11b168c3f371964c131528fcc92b365face2379
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5684976
Reviewed-by: Yuki Shiino <[email protected]>
Commit-Queue: Takashi Sakamoto <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1344569}
NOKEYCHECK=True
GitOrigin-RevId: 61f529a935b9053b0172c980a26e5389daf29aef
1 file changed