[identity] Add config for Blob service URL

Summary: Similiar to D12972. Added config option and env var to specify Blob service endpoint in Identity service

Test Plan:
Same as D12972
Cargo build. Played with clap CLI args in the terminal

Reviewers: varun, will, kamil

Reviewed By: kamil

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D13641