liblp: Add support for updateable partition groups.

This patch introduces a new "groups" table in the super partition
metadata. Each entry denotes a named partition group with a maximum
size. All partitions now belong to a group, and the total size of
partitions in a group must not exceed its maximum size. This is enforced
by MetadataBuilder.

There is also a "default" group with no size restriction. This is used
for one-off partitions that aren't restricted by updates, for example,
the scratch partition for overlayfs, or partitions created through
fastbootd.

Bug: 116817738
Test: liblp gtest
Change-Id: I7049ffd35d326e41e25d01b1748cb53a584783a7