Ability to pause tilt reload on demand

Occasionally, I need to stop work on an in-development branch to make a small amount of changes on a new branch. These changes don't necessarily need to run through tilt. At the same time, I'd like to quickly be able to switch back to what I was doing with minimal interruptions. Switching branches can have a number of bad side effects while tilt is running. E.g.

• I may have feature flags/tilt config that is incompatible with a new base branch
• The cluster may be in a specific hard to reproduce state

It would be nice to be able to temporarily pause resource reload (either from the terminal or GUI), change branches and make changes, then switch back to the original branch. and resume resource reloading.

A possible workaround today would be to instrument either a tilt_config option or environment variable and stop tilt prior to switching branches. A rough (untested) example is below.

tilt up
# in another terminal
tilt arg -- --preserve-resources all

# Tiltfile
if cfg.get('preserve-resources', []) != 'all':
   # Usual Tiltfile config
else:
   # No resources are defined, so tilt shouldn't try and delete anything
   pass

Review the proposed tilt arg -- --preserve-resources all workaround and the requested terminal or GUI control. Define how pausing should preserve the current cluster state while branches change, and how resuming should restore resource reloading without unwanted interruptions.