SavedScenario User Management via Engine

[louispt1]

Description

Implements a pass-through API that proxies SavedScenario user management requests to MyETM.
These changes, along with this Myetm PR allow users to manage roles on SavedScenarios via the Engine, and solidify the user coupling functionality so accounts are properly coupled based on the callback actions in MyETM.

Key changes:

• SavedScenarioUsersController with full CRUD endpoints for bulk user operations (/api/v3/saved_scenarios/:id/users)
• Scenario#copy_roles_from_saved_scenario to replicate user permissions when creating scenarios from presets
• ETEngine::MyETM module for a pre-configured Faraday client (mimicking setup in MyETM --> We can re-use the my_etm_client for other similar Faraday calls)
• Test coverage for the changes

Still to do

• Document SavedScenarioUsers CRUD endpoints

Relevant for this pyetm PR

Closes #1687