[lib][landing][web] Use getCommConfig to get API keys for Webpack builds

Summary:
Linear task: ENG-3229.

Now that we're using getCommConfig, these API keys can be specified with JSON files in keyserver/secrets instead of requiring patching package.json files with env var declarations.

Depends on D7354

Test Plan:

1. I created a valid keyserver/secrets/alchemy.json file
2. I added a console.log to print the value of alchemyKey from getConfigs in lib/webpack/shared.cjs
3. I made sure that it printed the right value when called from landing and web, in both dev and prod modes

I also made sure to set the corresponding env var on my production keyserver.

Reviewers: atul, varun, jon

Reviewed By: atul

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D7357