Add string algorithms: RemoveStars and ComplexNumberMultiply (#7275)

* Add RemoveStars and ComplexNumberMultiply string algorithms

* Add RemoveStars and ComplexNumberMultiply string algorithms

* Add unit tests for RemoveStars and ComplexNumber Multiply

* Fix checkstyle

* Remove redundant main method

* Move ComplexNumberMultiply to maths package and add input validation with tests

* Apply spotless formatting

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Co-authored-by: Deniz Altunkapan <deniz.altunkapan@outlook.com>