Feature - Add env variable support for alpha struct (#2375)

* added envsubstring package and added simple test cases.imple tests.

* added documentation

* added changelog entry

* added documentation to wrong file


.

* changed tests to ginkgo format

* update project to use better maintained library

* use defer to clear test variable after tests finished

* updated docs for the new package documentation and fixed bad english

* refactored function to "reduce" complexity.

* updated changelog for new version

updated readme

* minor formatting

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Co-authored-by: Haydn Evans <h.evans@douglas.de>
Co-authored-by: Joel Speed <Joel.speed@hotmail.co.uk>