[keyserver] Add a column to invite_links table with a blob holder

Summary:
Add a new column where a holder for invite link blob will be kept.

Depends on D9944

Test Plan: Start the keyserver and check if the migration adds a column

Reviewers: michal, bartek, rohan

Reviewed By: rohan

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D9960