Default environments per host imlementation

[create-issue-branch]

Changes

• each host implementation may have implemented function add_implementation_envs to modify environments before application launch (name of function can be changed...)
• implementation of the function may return totally different dict or just modify dictionary passed to function
• all required environments were moved to host implementation except of Resolve and few environment keys
• some of them are set only if are not set via settings some of them prepend paths in environment key
  • there is currently an issue with blender where is not possible to set BLENDER_USER_SCRIPTS which was not possible even before just mentioning this is not solved
  • harmony has LIB_OPENHARMONY_PATH which is always overriden without validation not sure if it's right
  • houdini has appended & symbol after it's environments not sure if it's requirement?
  • also not sure about few Maya's environments if should be defined in settings or not

closes #1405

[mkolar]

houdini has appended & symbol after it's environments not sure if it's requirement?

yes it is. it tells houdini to append it's own environments

harmony has LIB_OPENHARMONY_PATH which is always overriden without validation not sure if it's right

that should be correct yes.

[mkolar]

All is looking good. I've tested all hosts apart from TV paint