#2001 - Added signature generation via hmac-sha256

[k-macmillan]

Description

Added a new method: generate_callback_signature. Added a new version of the callback data generation method. While doing TDD I attempted to use the existing method to generate a callback payload but found it required far more setup than the new way. Duplicated the existing method but built it a little different to replicate what our clients will receive.

issue #2001

How Has This Been Tested?

TDD, also, unit test generated a key and it was used to verify the signature generation is consistent and meets hmac-sha256 standards.

Checklist

• I have assigned myself to this PR
• PR has an appropriate title: #9999 - What the thing does
• PR has a detailed description, including links to specific documentation
• I have added the appropriate labels to the PR.
• I did not remove any parts of the template, such as checkboxes even if they are not used
• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to any documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works. Testing guidelines
• I have ensured the latest main is merged into my branch and all checks are green prior to review
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules
• The ticket was moved into the DEV test column when I began testing this change

[MackHalliday]

Reviewed user story and PR, pulled down code locally to look at tests.

Changes make sense to me.

Approved.