Allow x-api-key for public rest api

[ashika112]

Description of changes

Allow x-api-key to pass through. We should check only custom authorization to override the IAM auth. This bring v6 in parity with v5 feature.

V5 Ref: https://github.com/aws-amplify/amplify-js/blob/b88df662d434465e25e797e68f9038819e8756a2/packages/api-rest/src/RestClient.ts#L163

Issue #, if available

#13310 #13378

Description of how you validated changes

• Manual testing
• Unit tests

Checklist

• [x] PR description included
• [x] yarn test passes
• [x] Tests are changed or added
• [ ] Relevant documentation is changed or added (and PR referenced)

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