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

[github-actions]

LocalStack Community integration with Pro

    2 files  ±0      2 suites  ±0   1h 36m 33s ⏱️ +7s
2 932 tests ±0  2 638 ✅ ±0  294 💤 ±0  0 ❌ ±0 
2 934 runs  ±0  2 638 ✅ ±0  296 💤 ±0  0 ❌ ±0 

Results for commit 91d3dea. ± Comparison against base commit 7917b4a.

♻️ This comment has been updated with latest results.