Vesting merge_schedules should be added to the NonTransfer proxy filter

[emostov]

paritytech/substrate#9202 + #3407 will add the ability to merge two vesting schedules. This call should be added to the NonTransfer proxy filter.

The main attack with merge schedules I can think of is to send an account a schedule that starts and ends at BlockNumber::max_value and then merge all there schedules with that one, which will make sure they can't unlock funds until block height hits BlockNumber::max_value.

After some quick discussion with @shawntabrizi I think this risk profile is in line with some other permitted calls for NonTransfer.