Ensure we only depend on FoundationEssentials in JSON mode

### Expected behavior

N/A

### Actual behavior

N/A

### Steps to reproduce

N/A

### If possible, minimal yet complete reproducer code (or URL to code)

_No response_

### What version of this project (`swift-aws-lambda-runtime`) are you using?

N/A

### Swift version

N/A

### Amazon Linux 2 docker image version

_No response_