NPA-4907 Enforce Single Performer Item

[JackPlowman]

Pull Request

Ticket Link

https://nhsd-jira.digital.nhs.uk/browse/NPA-4907

Description/Change Summary

• Set MaxItems of 1 for Consent performer
  Note: This change applies to both the GET /Consent response body and POST /Consent request body

How to test?

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.

Post-merge

After merging and deploying changes to the sandbox, Postman collection or spec examples please run the Run Postman collection workflow.

This will run the tests within the collection to check that the sandbox is working as expected once deployed.

[github-actions]

This branch is work on a ticket in the NHS Digital NPA JIRA Project. Here's a handy link to the ticket:

NPA-4907