[OpenMP][libomptarget] Improve NextGen plugin interface for initialization

This patch modifies the PluginInterface to define functions for initializing
and deinitializing GenericPluginTy instances instead of using the constructor
and destructor. This way, we can return errors from these functions. Also, it
defines some functions that each plugin should implement for creating
plugin-specific objects.

This patch prepares the PluginInterface for the new AMDGPU NextGen plugin.

Differential Revision: https://reviews.llvm.org/D138625