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

[chipweinberger]

DevTools should support custom data and logic. For example:

• inspect state of my internal app database
• run custom debug commands in my app, and see their results

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

[iapicca]

@chipweinberger I suspect this issue should be opened here

[danagbemava-nc]

@iapicca is right. Transferring to the devtools repo.

[kenzieschmoll]

Closing as a duplicate of #1632