Issue: 2236 - adds an option to append CA certificates (#2237)

* adding append option for custom CA certs

* updated test for changed GetCertPool signature, added testing to check functionality of empty and non-empty store

* adding legacy options as well

* update associated documentation

* fixing code climate complaints - reduce number of return statements

* Apply suggestions from code review

Changes caFilesAppend (and variants) to useSystemTrustStore

Co-authored-by: Jan Larwig <jan@larwig.com>

* Apply suggestions from code review

Fixes extra whitespaces and grammar.

Co-authored-by: Koen van Zuijlen <8818390+kvanzuijlen@users.noreply.github.com>

* fix indentation

* update changelog

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Co-authored-by: Jan Larwig <jan@larwig.com>
Co-authored-by: Koen van Zuijlen <8818390+kvanzuijlen@users.noreply.github.com>
Co-authored-by: Joel Speed <Joel.speed@hotmail.co.uk>