fix(parser): allow SQS envelopes to handle non-JSON strings

[dreamorosi]

Summary

Changes

Please provide a summary of what's being changed

This PR updates the SqsEnvelope and SnsSqsEnvelope so that they can handle messages other than JSON strings, i.e. plain text strings like "hello world".

The PR is still WIP and I will update the description once it's done.

To complete this PR I also had to refactor the Middy.js middleware unit tests since they were using the SQS envelope I was touching. Because of this I took the opportunity to simplify & standardize them so that they follow the same conventions as the rest of the project.

Please add the issue number below, if no issue is present the PR might get blocked and not be reviewed

Issue number: fixes #3510

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[am29d]

Thanks for the PR! Just a minor question on reusing createError helper.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
12.3% Duplication on New Code

See analysis details on SonarQube Cloud