Support global lists in SAM templates

**Describe the solution you'd like**

Lists are not currently supported by our global handling code. Specifically, any global lists are [additive to any lists in resources](https://docs.aws.amazon.com/serverless-application-model/latest/developerguide/sam-specification-template-anatomy-globals.html#sam-specification-template-anatomy-globals-overrideable). Addition to https://github.com/aws/aws-toolkit-vscode/pull/1493 .

**Additional context**

Relatively low priority as we're not currently handling any SAM properties involving lists programmatically within the toolkit (we just pass it to the SAM CLI which handles it on its own).

Start by reviewing the existing global handling code and the changes in aws-toolkit-vscode pull request #1493. Trace how SAM template globals are merged with resource lists; done means global lists are supported additively while preserving lists defined on resources.

Written by the indexing model from the issue text.