Refactor RegexFilter to use a plugin builder, restore configurable re…

[ramanathan1504]

Fixes #3086

This pull request refactors the RegexFilter to use a plugin builder pattern, restores support for configuring regex patternFlags via a comma-separated attribute, and improves robustness around the useRawMsg option. It also adds tests and documentation for the new pattern flags feature.

RegexFilter Refactor and Feature Restoration:

• Refactored RegexFilter to use a plugin builder (Builder class), simplifying construction and improving maintainability. The builder now uses a primitive boolean for useRawMsg to avoid null-unboxing issues. [1] [2]
• Restored and improved support for configurable regex patternFlags via a comma-separated string attribute (e.g., patternFlags="CASE_INSENSITIVE"), replacing the previous, more cumbersome approach.

Testing Improvements:

• Added a new test (testPatternFlagsFromConfiguration) in RegexFilterTest.java to verify that pattern flags are correctly loaded from configuration files.
• Added a corresponding test configuration file RegexFilterPatternFlagsTest.xml to exercise the new pattern flags feature.

Documentation and Changelog:

• Updated the manual to document the patternFlags attribute, listing all supported flag values and usage.
• Added a changelog entry describing the refactor, restoration of patternFlags, and the builder improvements.

Other:

• Bumped the OSGi version for the filter package to 2.26.0 to reflect the new feature.