Add a "VestingOrigin" to allow for the creation of vesting schedules indefinitely

[lsaether]

It is foreseeable that a project will want to apply a VestingSchedule to accounts after the creation of the initial chain spec. Currently, there is not a way to do this without setting vesting in the chain spec or by implementing a new pallet.

The refactor to balances in #4649 separates vesting into its own pallet. I propose an addition of a new dispatch that allows for a permissioned account (the VestingOrigin) to set new vesting schedules. This could be combined with a transfer of funds so that the vesting schedule cannot be completely arbitrarily set by the vesting origin, but only if it transfers the funds that would be vested at the same time.

Concretely, this is a problem for the W3F Grants team that would like to continue awarding grants with a schedule attached. However, other projects building on Substrate may benefit from this addition as well.

cc @shawntabrizi