* Add keepFiles support for time-based log rolling
Added <keepFiles> option to "roll-by-time" log mode, allowing configuration of how many rolled log files to retain. Updated documentation, XmlServiceConfig parsing, and TimeBasedRollingLogAppender to support and enforce file retention. Old log files are now purged automatically when the limit is exceeded.
* test: add unit tests
Changed PurgeOldFiles to internal for test access. Added two unit tests to verify file purging behavior in TimeBasedRollingLogAppender.
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Co-authored-by: Ciou, Mao <mao.ciou@garmin.com>
