Add private github repositories as custom plugins easily

[focusaurus]

We have some implementations that have integrated private reaction plugins using a fork of reaction and adding their plugins to that fork using git submodules. This does work but comes with significant friction:

• 1 full PR cycle to land your feature in your plugin
• a separate full PR cycle to update the submodule in the reaction fork
• Some nuisances inherent in submodules like
  • spuriously-modified package-lock.json files
  • lots of noise in git status with m and M flags

I initially thought I had the beginnings of a proposed alternative, but as I thought it through I realized it was essentially "roll your own git submodules".

( CC @machikoyasuda )

[focusaurus]

For reference: Mastering git submodules and Mastering git subtrees