Fix header forwarding in request signing in proxy server

[kmcginnes]

Description

This change fixes an issue where the request signing would not properly merge the original request and the signed request. This resulted in certain headers and request options left out of the final request that is sent to the database.

I've also reorganized the code and added a bevy of tests around the request signing.

Validation

• Tested against Neptune with IAM enabled
• Tested against public RDF endpoint

Related Issues

• Resolves Some headers are not forwarded to database request from proxy server #1170

Check List

• I confirm that my contribution is made under the terms of the Apache 2.0
  license.
• I have run pnpm checks to ensure code compiles and meets standards.
• I have run pnpm test to check if all tests are passing.
• I have covered new added functionality with unit tests if necessary.
• I have added an entry in the Changelog.md.

[kmcginnes]

Marking as draft. While the bulk of this change is still good, I have some other cleanup to do first. And this is not required for the SPARQL query editor work.