v0.0-594-g5ae8821: Attach alignment policy to each alignment group.

This supports extracting different of types alignment groups
from a single token partition range.

Alignment handler map value-type is now a generic function, allowing greater
flexibility in how alignment groups are created, replacing previous struct
entry.

Legacy adapter updated to apply one AlignmentPolicy uniformly to all alignment
groups.  This is not suitable for when we want each group to have its own
policy.

No functional change.

PiperOrigin-RevId: 331873028