CCM-12600: Add sha256 hash to letters

[RossFaulds171]

Description

Letters need to receive the Sha 256 hash of the letter pdf file, and to check that this
matches with the actual pdf to ensure no tampering has occurred

Add hash retrieve to letter retrieval api endpoints
Add hash to sandbox test
Add unit tests
Modify open api spec
Modify component tests

Context

We receive a hash of the generated pdf file on an event which is stored as meta data.
However, the api doesn't retrieve the hash or validate that the hash matches the actual file so that
any tampering could be detected.

Type of changes

• Refactoring (non-breaking change)
• x] New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would change existing functionality)
• Bug fix (non-breaking change which fixes an issue)

Checklist

• I am familiar with the contributing guidelines
• I have followed the code style of the project
• I have added tests to cover my changes
• I have updated the documentation accordingly
• This PR is a result of pair or mob programming

DT3-Specific Checklist

• If I have added a new resource (SQS, Lambda, Gateway, DDB table, etc), I have created the appropriate alarms

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Sensitive Information Declaration

To ensure the utmost confidentiality and protect your and others privacy, we kindly ask you to NOT including PII (Personal Identifiable Information) / PID (Personal Identifiable Data) or any other sensitive data in this PR (Pull Request) and the codebase changes. We will remove any PR that do contain any sensitive information. We really appreciate your cooperation in this matter.

• I confirm that neither PII/PID nor sensitive data are included in this PR and the codebase changes.