[Feature] DevTools should be 3rd party extensible, with custom tools and data

[chipweinberger]

DevTools should support custom data and logic. For example:

• inspect state of my internal app database
• a command line interface for my app, that I can interact with from my PC

Of course, this code would be highly custom to my app, and can only be achieved if DevTools is extensible by 3rd parties

Motivation (Versus In-App Debug Panel):

• dev tools offers lots of screen realestate
• dev tools has already solved the problem of getting custom data off the phone over the lightning cable

[kenzieschmoll]

Closing as a duplicate of #1632. You can follow along there for updates to the DevTools plugin framework that is under design.

[chipweinberger]

Thanks @kenzieschmoll ! =)