Method to align/merge dimensional/symbolic identifiers between two FlatAffineConstraints

- add a method to merge and align the spaces (identifiers) of two
  FlatAffineConstraints (both get dimension-wise and symbol-wise unique
  columns)
- this completes several TODOs, gets rid of previous assumptions/restrictions
  in composeMap, unionBoundingBox, and reuses common code
- remove previous workarounds / duplicated funcitonality in
  FlatAffineConstraints::composeMap and unionBoundingBox, use mergeAlignIds
  from both

PiperOrigin-RevId: 236320581