DCMAW-9802: Refactor of Client Credential service

[sandy-may]

​DCMAW-9802

What changed

Refactor of the client credential service:

• Renamed to client registry service
• Expose two public methods on the class for retrieving registered client details

Why did it change

• There is data in the client registry that doesn't relate to client credentials
• The lambdas had to orchestrate the service adding extra complexity to the lambda handler. This simplifies it so the lambda only knows what it needs to know to perform its job.

Checklists

• There is a ticket raised for this PR that is present in the branch name
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• Demo to a BA, TA, and the team.
• Update README with any new instructions or tasks