[keyserver][lib] Add COMM_WALLETCONNECT_KEY to Webpack build

Summary:
This is similar to how we handle COMM_ALCHEMY_KEY. It will be exposed to the Webpack build through the Dockerfile, and additionally it will be exposed to keyserver. (We don't need it there though.)

This addresses ENG-2969

Test Plan: Will test in combination with following diffs

Reviewers: atul

Reviewed By: atul

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D6670