Support for client secret file. (#355)

* added ClientSecretFile in ProviderData

* add documentation notes on client secret file

* added Changelog entry for Client Secret File PR

* fixing configuration.md

* addressing PR issue of ClientSecret property naming

* Update providers/provider_data.go

Co-Authored-By: Joel Speed <Joel.speed@hotmail.co.uk>

* corrected changelog entry

* fixed typo in GetClientSecret

Co-authored-by: Joel Speed <Joel.speed@hotmail.co.uk>