[keyserver] add migration type to migrations

Summary:
This introduces the `MigrationType` which we can use to specify whether we want to wrap a migration in a transaction and block secondary nodes.

Depends on D13208

Test Plan: Tested later in stack

Reviewers: varun, ashoat

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D13209