Fetch endpoints.json dynamically instead of storing in repo

[awschristou]

Describe the solution you'd like

The codebase should not contain endpoints.json. It should pull it down as part of the npm install step, so that the toolkit builds always embed/contain the most recent version.

[justinmk3]

It should pull it down as part of the npm install step, so that the toolkit builds always embed/contain the most recent version.

Note that this may prevent a "reproducible build". If we had a versioned URI for the endpoints.json file that could be a cheap way to store the expected endpoints.json in the Toolkit source.