[lib] Introduce blobURI multimedia message field

Summary:
First part of ENG-3966. Details in the Linear task.
Replaced the holder field for Encrypted{Image,Video} with union holder | blobURI and updated validators accordingly

Added @michal because I made changes to validators.

Test Plan:
This stack is tested altogether, CI will fail at this point due to Flow breaking changes which will be solved in subsequent diffs.

Actual test plan in the last diff.

Reviewers: ashoat, atul, michal

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D8370