[web] Pass platform details to worker

Summary:
After rebasing on master, D11342 (child diff) will now need to have access to platform details inside olmAPI. Because of that I changed the approach where the platform details is passed only in identity client creation and instead it's now passed during web worker initialization and saved as a global variable.

Depends on D11341

Test Plan:

• Added a console log on processing worker request, made sure that it displayed current platform details after init
• Check if calls to identity service work

Reviewers: kamil, tomek

Reviewed By: kamil

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D11385