[web] Add hook to generate placeholder image

Summary:
This diff adds a hool that generates placeholder data URL from given thumbhash string. It also decrypts the thumbhash if encryption key is provided.

The operation is synchronous for non-encrypted thumbhash and asynchronous for encrypted ones.

Depends on D7900

Test Plan: Tested together with the next diff.

Reviewers: ashoat, atul, ginsu, inka

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D7901