feat: add optimized Digit DP template and unit tests (#7430)

* feat: add optimized Digit DP template and unit tests

* test: add test cases for max target sum and memoization hit to achieve 100% coverage

* style: fix indentation and code formatting for clang linter compliance

* fix: remove checkstyle inner assignment in solve method

* fix: remove checkstyle inner assignment in solve method -newline at end