feat(toolkit-lib): bootstrap action

[iankhou]

Fixes aws/aws-cdk#33180. Typed return will be included in a follow-up PR for #192.

Description:

Bootstrap Action for the CDK Toolkit.

Testing:

• Unit test coverage for:
  • Bootstrap stack creation
  • No-op (no stack changes)
  • Generic bootstrap error
  • Permissions error during bootstrap
• Bootstrapped an environment in a test account using this code

---

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license

[mrgrain]

This action has a slightly different syntax. It's an environment based action that takes in a list of environment strings.

There is an option to read environments from a provided cx (see defineEnvironments` in the current code). But I'm wondering if we should instead provide this as a helper function that can be called "outside".

[iankhou]

Added userEnvironmentSpecs, which, when empty, will select all environments from all stacks (like deploy).

[iankhou]

In the new interface, the function must be called with either environments or cx.

[iankhou]

Just kidding, I created a new class called BootstrapEnvironments that takes in either a cx or list of envs.