MDEV-28442: Hashicorp: refactoring to wrap static variables into a class

This commit contains changes to refactor the the Hashicorp plugin code
which hides all variables previously declared as "static" and which are
not user-visible parameters into a special class that contains all the
plugin's dynamic data. This was done primarily to significantly simplify
the code of the initialization and deinitialization functions, which
previously contained a large number of gotos and complex branching
conditions to control memory deallocation.