[keyserver][lib][web] Make it possible to call getCommConfig from Webpack config

Summary:
Linear task: ENG-3229.

We have three configs that we need to be accessible from the Webpack build:

1. COMM_ALCHEMY_KEY
2. COMM_WALLETCONNECT_KEY
3. Coming soon: config for identity service

Currently it's a huge pain to configure these in a dev environment. keyserver configs can be imported via getCommConfig, but it doesn't work in other environments.

This diff makes it possible to call getCommConfig from web.

Depends on D7351

Test Plan:
I tested with this patch: https://gist.github.com/Ashoat/487aa5a611e84f1751d7c60d82b8e4f9

I made sure that it printed the contents of commapp_url.json when called from landing and web, in both dev and prod modes.

Reviewers: atul, varun, jon

Reviewed By: atul

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D7354