Add extensionActivationState endpoint to the devtools server

[kenzieschmoll]

This endpoint manages reading and writing to/from the devtools_options.yaml file. This file will store DevTools extension activation states and will live in a project's root directory (similar to analysis_options.yaml).

Example devtools_options.yaml file:

extensions:
  - foo: true
  - bar: false

In this scenario:

• the devtools extension for package:foo has been enabled by the user
• the devtools extension for package:bar has been disabled by the user
• the devtools extension for some other package:baz has been neither enabled nor disabled (meaning that DevTools should prompt the user to activate).

This API will be consumed by DevTools app in a follow up PR.

work towards #1632