[web][native] Download invite link blob and use it as an override

Summary:
Download a blob, parse it and use its content as a keyserver override.

Depends on D10311

Test Plan: Open a link on web for which a blob is downloaded and check if a blob is downloaded and its content is used correctly.

Reviewers: michal, bartek, rohan, inka

Reviewed By: bartek

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D10338