correctly handle non backend outputs from stack metadata

[rtpascual]

Problem

Using CDK nag or doing backend.stack.addMetadata() causes ZodError when generating the amplify_outputs.json file. The ZodError is occurring when we are parsing the stack metadata in order to fetch backend outputs and the parsing is not expecting non backend output entries.

Issue number, if available:

Changes

Filter out non backend output entries in stack metadata before parsing.

Corresponding docs PR, if applicable:

Validation

Unit tests

Checklist

• If this PR includes a functional change to the runtime behavior of the code, I have added or updated automated test coverage for this change.
• If this PR requires a change to the Project Architecture README, I have included that update in this PR.
• If this PR requires a docs update, I have linked to that docs PR above.
• If this PR modifies E2E tests, makes changes to resource provisioning, or makes SDK calls, I have run the PR checks with the run-e2e label set.

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