Feature Request: Allow cancellation of running applier/destroyer tasks

Currently, the baserunner handles context cancellation, but the only task it can currently interrupt is WaitTask, which it does by calling private methods (eww).

As a user who uses the global context for cancellation/timeout, I want to be able to interrupt the applier/destoryer and have it exit quickly, without finishing the ApplyTask/PruneTask.

Solution:
- Pass context to tasks by adding it to the TaskContext
- Update all tasks to handle context.Done() and exit early
- Update all tasks to pass the context to all network calls, so they also can be cancelled (most already take a context)
- Add a Task method to handle status events, so WaitTask can exit early.

Start by tracing TaskContext and the baserunner's handling of context cancellation, then inspect WaitTask, ApplyTask, PruneTask, and the other tasks. Check which network calls already accept a context and how status events are handled. Done means applier and destroyer tasks exit promptly on cancellation, network calls receive the context, and WaitTask no longer depends on private methods.

Written by the indexing model from the issue text.