[lib] Use response validators in multiple-holder calls

Summary:
Used types and validators from D13655 to make Blob fetch calls more type safe.

Depends on D13655

Test Plan: Flow. Verified that blob service calls using these functions succeed.

Reviewers: kamil, tomek

Reviewed By: kamil

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D13656