MINIFICPP-2162 Use className() instead of getClassName()

[fgerlits]

https://issues.apache.org/jira/browse/MINIFICPP-2162

Replace usages of getClassName() -> std::string with className() -> std::string_view. This led to some other const std::string& to std::string_view changes in the parameter types of some functions.

I have also removed a static_assert in the className() implementation, because it caused CLion to show bogus errors, probably due to a confusion between gcc (used to compile the code) and clang (used to analyze the code). If the condition fails, we will still get a compile error, just with a less useful error message.

---

Thank you for submitting a contribution to Apache NiFi - MiNiFi C++.

In order to streamline the review of the contribution we ask you
to ensure the following steps have been taken:

For all changes:

• Is there a JIRA ticket associated with this PR? Is it referenced
  in the commit message?

• Does your PR title start with MINIFICPP-XXXX where XXXX is the JIRA number you are trying to resolve? Pay particular attention to the hyphen "-" character.

• Has your PR been rebased against the latest commit within the target branch (typically main)?

• Is your initial contribution a single, squashed commit?

For code changes:

• If adding new dependencies to the code, are these dependencies licensed in a way that is compatible for inclusion under ASF 2.0?
• If applicable, have you updated the LICENSE file?
• If applicable, have you updated the NOTICE file?

For documentation related changes:

• Have you ensured that format looks appropriate for the output in which it is rendered?

Note:

Please ensure that once the PR is submitted, you check GitHub Actions CI results for build issues and submit an update to your PR as soon as possible.