correctly handle stack argument for generate schema command

Problem

When passing stack option for generate schema-from-database command, secret client getSecret is expecting backend identifier to be like https://github.com/aws-amplify/amplify-backend/blob/main/packages/plugin-types/src/backend_identifier.ts#L17 but it is resolved to { stackName: '<stack>' }. So we fail to convert the backend identifier to parameter path strings for connection URI and SSL Cert secrets.

Issue number, if available:

If stack is passed, we correctly convert to BackendIdentifier before getting secrets.

Corresponding docs PR, if applicable:

Updated test.

[ ] 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.

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