Allow setting/unsetting the active contexts programmatically

Problem/Motivation

In D7 we could set an active context during runtime in a custom module using context_set_context()
This does not seem possible in the D9/10 version?

Proposed resolution

Create public methods in the ContextManager service to allow for setting and unsetting an active context.

setActiveContexts($context_names);
unsetActiveContexts($context_names);
isActiveContext($name);

ex:

// Set context_1 and context_2 to be active, if they exist.
\Drupal::service('context.manager')->setActiveContexts(['context_1', 'context_2']);