Revert "chore: remove legacy schema validation code (#3257)"

[hoffa]

Issue #, if available

Description of changes

This reverts commit 6a16639.

Noticed SamTemplateValidator is used in SAM CLI: https://github.com/aws/aws-sam-cli/blob/84184943aeec2423f1071a88297f7722edf6cbf2/samcli/lib/samlib/wrapper.py#L160

While it doesn't strictly break anything (the function calls still exist), creating this PR as heads-up to better determine whether the validation is desired, but can likely be closed again.

The reverted uses hand-written schemas to perform partial template validation; I'm not sure what the value is compared to just doing the transform.

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.