[keyserver] Add pinned_count column to MariaDB threads table

Summary:
We need to include a pinned_count column to the threads table in MariaDB in order to include it in the thread info in a later diff. Modifications to the client db and the new thread types / persist migration will come in the next two diffs

Part of
https://linear.app/comm/issue/ENG-3396/store-the-number-of-pinned-messages-in-threadinfo

Depends on D6924

Test Plan:
Run the migration and confirm that the pinned_count column appears in MariaDB

Reviewers: atul, ginsu, tomek, ashoat

Reviewed By: ashoat

Differential Revision: https://phab.comm.dev/D7199