feat: unified SAM/CloudFormation schema

[hoffa]

Issue #, if available

Description of changes

Adds new unified.schema.json that includes both SAM and CloudFormation resources.

We need control over the CloudFormation schema, so that e.g. we can allow additionalProperties such as #2772 on them.

Other notes:

• Eventually will rename schema.json to sam.schema.json, and unified.schema.json to schema.json so that existing users of the schema will automatically get the improvements.
• The CloudFormation schema doesn't support intrinsic functions (see SAM/CFN json schema: handle of intrinsic functions aws-toolkit-vscode#2596).
• We should add some automation to automatically update the schema.

Description of how you validated changes

Checklist

• Adheres to the development guidelines
• Add/update transform tests
  • Using correct values
  • Using wrong values
• Add/update integration tests

Examples?

Please reach out in the comments if you want to add an example. Examples will be
added to sam init through aws/aws-sam-cli-app-templates.

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