Add native support for deletion protection - #440

This supersedes #418 and keeps its four commits from @mnapoli as they were, rebased onto 4.x, followed by one commit with the changes that came out of review.

Deletion protection now lives in its own internal plugin, mirroring provider.pruneFunctionVersions: it validates the configuration before the deploy and sets the stack's termination protection on after:deploy:deploy, which covers every terminal deploy path, including deploys skipped because nothing changed, while leaving rollback and deploy function alone, so those commands no longer need cloudformation:UpdateTerminationProtection. Values that slip past schema validation under configValidationMode: warn or off fail with INVALID_DELETION_PROTECTION_CONFIG before anything is uploaded instead of silently disabling protection, an SDK failure after an otherwise successful stack update is reported as such, and a first change-set deploy that ends with an empty change set no longer protects a stack left in REVIEW_IN_PROGRESS.

The remove guard warns and continues when DescribeStacks is denied, its message covers the stages form and the AWS CLI escape hatch, and the ECR repository probe runs concurrently again. The docs spell out that unlisted stages are actively unprotected, which differs from the plugin this replaces, plus the IAM permissions, the first-deploy window and the deploy --package caveat. The schema moved to a named awsDeletionProtection definition and the unrelated type reflows were dropped.