NIFI-15088 Refactor Login Provider loading with DOM instead of JAXB

[exceptionfactory]

Summary

NIFI-15088 Refactors framework loading of the login-identity-providers.xml configuration using the XML DOM API instead of JAXB.

The XML schema for Login Identity Providers is straightforward with a small number of elements and attributes. Moving away from JAXB avoids both build dependencies on the Maven plugin for compiling Java classes from an XSD, and runtime dependencies on JAXB for loading XML.

The existing unit tests exercise the Login Identity Provider loading process, validating these changes.

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  • JDK 21
  • JDK 25

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[pvillard31]

Thanks @exceptionfactory, this looks good and a much better approach! Tested this PR and #10417 together.