[services][terraform] Add new blobs table definition

Summary:
Defined a new table for blobs in terraform, according to ENG-4267 design (Notion doc attached).

Also added constant names for the new table (I think it's easier to review when these are together).

Test Plan: terraform plan and apply.

Reviewers: michal, jon, varun

Reviewed By: varun

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D8439