[AB2D-6148] increase ab2d-fhir code coverage

[coilysiren]

🎫 Ticket

https://jira.cms.gov/browse/AB2D-6148

🛠 Changes

• Adds a FhirVersion.getClassFromName function, which is patterned off of the other methods in that file. It is a simple class that calls Class.forName(...) and logs all exceptions.
• Changes ExtensionUtils to use FhirVersion.getClassFromName. This means that ExtensionUtils can get rid of a few try ... except blocks that were functionally dead code. Those lines were dead code because they were calling Class.forName(...) on static strings... so they were never going to raise an exception. Hence, it was impossible to test them in their previous form.
• Adds tests for the above.

ℹ️ Context

The goal of this PR is to increase test coverage for our code. At present ExtensionUtils has the lowest test coverage of all the files in this folder. Unfortunately, none of the code is "easy" to test, so I'm having to resort to code changes to get test coverage any higher.

The code I added has 100% test coverage 😎 . Additionally, FhirVersion and ExtensionUtils now both have 100% test coverage as well.

[Rwolfe-Nava]

Minor suggestion / question regarding assertions, but otherwise looks good to me.