CFn: Improve stack set support

[simonrw]

Motivation

I updated our copilot-local repo to the latest upstream commit, and it did not work against LocalStack any more. I am not 100% sure the current commit works either.

The initial failure reason was our StackSet support in CloudFormation.

Changes

• Update get_template_body to allow use of the previous stack (UsePreviousStack)
• Expand on the StackSet entity mostly allowing access to the template body for use by StackInstances
• Look up stack sets when retrieving template summaries
• Merge StackInstance parameter overrides with StackSet parameter definitions
• Introduce convert_in_place_at_jsonpath for deep nested structure in place type conversion
• Expand on test_create_stack_set_with_stack_instances test to support parameter overrides

Testing

I have been testing with my fork of copilot-local which has a few changes to the LocalStack fork of copilot:

• the localstack branch has been rebased onto the latest tagged release (v1.33.3 at the time of writing)
• the manifest parsing has been improved to correctly handle escaped newlines, which cause invalid yaml parsing

As of 2024-06-07 I had not yet completed the work required to fix copilot-local. I think I was close, but not there yet.

[github-actions]

LocalStack Community integration with Pro

    2 files  ±0      2 suites  ±0   1h 35m 55s ⏱️ + 1m 7s
3 028 tests ±0  2 693 ✅ ±0  335 💤 ±0  0 ❌ ±0 
3 030 runs  ±0  2 693 ✅ ±0  337 💤 ±0  0 ❌ ±0 

Results for commit 9424427. ± Comparison against base commit 2f05d65.

♻️ This comment has been updated with latest results.