VED-807 Refactor backend

[JamesW1-NHS]

Summary

Note to Testers: There is no change in functionality here. Refactoring only.

• Routine Change

This is the refactoring of the backend lambda into the lambdas/ and shared/ folders.

Some components have been tidied up and refactored from other lambdas in the course of this. Examples:

• errors.py files: only errors which are common to two or more lambdas are in shared/; all others are in errors.py files local to their lambdas.
• All instances of an s3 client are now singletons from common.clients.get_s3_client
• Ditto redis client: common.redis_client.get_redis_client
• Some duplicated utility functions have been refactored into shared e.g. move_file()

Reviews Required

• Dev
• Test
• Tech Author
• Product Owner

Review Checklist

ℹ️ This section is to be filled in by the reviewer.

• I have reviewed the changes in this PR and they fill all or part of the acceptance criteria of the ticket, and the code is in a mergeable state.
• If there were infrastructure, operational, or build changes, I have made sure there is sufficient evidence that the changes will work.
• I have ensured the changelog has been updated by the submitter, if necessary.

[github-actions]

This branch is working on a ticket in the NHS England VED JIRA Project. Here's a handy link to the ticket:

VED-807

[sonarqubecloud]

Quality Gate passed

Issues
10 New issues
0 Accepted issues

Measures
0 Security Hotspots
92.7% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud