CCM-12600: Add sha256 to letters

[RossFaulds171]

Description

Add sha256 to get letters and get letter api endpoints, and ensure stored in db as well.

Change Letter schema to have sha256Hash defined.

Context

Currently the Sha256 hash is not exposed anywhere, and therefore cannot be used for any purposes, such as pdf file tampering etc.

Type of changes

• Refactoring (non-breaking change)
• 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.