We add all Role policy statements to the Inline policy, which
has a maximums size of 10k. Especially when creating CDK Pipelines
that deploy to a lot of environments, the list of Role ARNs
the Pipeline should be allowed to assume exceeds this size.

Roles also have the ability to have Managed Policies attached
(10 by default, 20 with a quota increase), each of them can be 6k
in size. By spilling over from inline policies into Managed Policies
we can get a total of 70k of statements attached to reach Role.

This PR introduces `IComparablePrincipal` to be able to value-compare
two principals: since we want to merge first before we split (to get the
most bang for our buck), we now need to do statement merging during the
prepare phase, while we are still working on the object graph (instead
of the rendered CloudFormation template).
* That means statement merging had to be modified to work on
  PolicyStatement objects, which requires being able to compare
  Principal objects.

Closes #19276, closes #19939, closes #19835.