A simple/model implementation of being able to obtain harvested metadata records unparsed, as Strings

[landreev]

• Closes Feature request: I'd like to be able to instruct my OAI client to return harvested metadata records unparsed, as Strings #284 (maybe?)

As the title says, this is a "model" implementation - primarily to illustrate what I want the final result to be on the application side.
I expect that it will likely need to be implemented differently before the PR can be merged. I'm happy to work on a cleaner/better implementation.

[sonarqubecloud]

Quality Gate failed

Failed conditions
37.5% Coverage on New Code (required ≥ 65%)

See analysis details on SonarQube Cloud

[pdurbin]

I didn't run this but it seems reasonable to me.