feat!: use toolkit-lib for diff (#125)

[corymhall]

This PR refactors the diff and assembly processing to use the new
@aws-cdk/toolkit-lib library.

This allows us to greatly simplify the logic that we have to maintain
and offload more work to the core library.
For example, this allows us to use the default CDK authentication
instead of trying to replicate it ourselves (see #62).

Couple of other changes that I've included in this PR since v2 allows me
a chance to make breaking changes.

BREAKING CHANGE: several breaking changes with details below

There are several breaking changes in this release.

1. Replace noDiffForStages with stackSelectionStrategy &
   stackSelectorPatterns
   This uses the native selection stack filtering capability of
   toolkit-lib and should be a more robust option for users to filter
   stacks. stackSelectorPatterns also uses a multi-line input instead of
   a comma delimited string input. To migrate from noDiffForStages to
   stackSelectorPatterns you can do this:

# from this
- name: Diff
   uses: corymhall/cdk-diff-action@v1
   with:
     noDiffForStages: "Stage1,Stage2"
     githubToken: ${{ secrets.GITHUB_TOKEN }}

# to this
- name: Diff
   uses: corymhall/cdk-diff-action@v2-beta
   with:
     stackSelectorPatterns: |
       !Stage1/*
       !Stage2/*
     githubToken: ${{ secrets.GITHUB_TOKEN }}

2. The default diffMethod is changed to change-set to match the cdk
   default behavior. This also changes the IAM Role used for diff from the
   lookup-role to the deploy-role. To keep the old behavior you can
   specify diffMethod: template-only
3. allowDestroyTypes and noFailOnDestructiveChanges input types were
   changed from a comma delimited string to a multi-line string.

# from this
- name: Diff
   uses: corymhall/cdk-diff-action@v1
   with:
     noFailOnDestructiveChanges: "Stage1,Stage2"
     githubToken: ${{ secrets.GITHUB_TOKEN }}

# to this
- name: Diff
   uses: corymhall/cdk-diff-action@v2-beta
   with:
     noFailOnDestructiveChanges: |
       Stage1
       Stage2
     githubToken: ${{ secrets.GITHUB_TOKEN }}

Closes #44
Fixes #62

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