[lib][web] Add identity service config to Webpack build

Summary:
In order to be able to access the identity service config we configure keyserver with in web, we need to include it in the Webpack build.

I initially tried doing this with two separate webpack.DefinePlugin invocations, but they conflicted:

WARNING in DefinePlugin
Conflicting values for 'process.env'

Test Plan:
I tested by running a local identity service and patching my checkout with the following: https://gist.github.com/Ashoat/d3bb12e6f10ddda90bb785ff0476ce9a

Note that the identity service returned an error, but the error was received by the client so I think things are working. Identity service error is tracked in ENG-3631

Reviewers: varun, atul, jon

Reviewed By: atul

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D7371