fix: consolidate backend secret custom resources into one

[awsluja]

Problem

This consolidates secret fetcher custom resources into one.

A single custom resource will be created, instead of a resource for each secret.

Issue number, if available:
#1797
#1825

Changes

Corresponding docs PR, if applicable:

Validation

Tests updated.

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.

[changeset-bot]

🦋 Changeset detected

Latest commit: ac67ec4

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@aws-amplify/backend	Patch

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[hoangnbn]

great improvement!

[awsluja]

closing in favor of #2011