IF: Add core contract actions to support Savanna

Change Description

Add core contract actions to support Savanna. For details please see https://github.com/AntelopeIO/reference-contracts/issues/24.

Switchover: Action to permanently transition to HotStuff consensus.

Require the authority of the contract itself
Set a flag altering the behavior of the update_elected_producers function
Establish a new finalizer set (introduced by the Instant Finality protocol feature)
Abort if an insufficient number of the top 21 block producers have registered a BLS finalizer key

Register Finalizer Key: Action to register a finalizer key.

Registered BPs Only: must be a registered block producer
Multiple Registered Finalizer Keys: A registered block producer can have multiple registered finalizer keys.
Proof of Possession: The finalizer key to register must be accompanied by a valid Proof of Possession signature.
Activate on First Key: If this is the first registered finalizer key of the block producer, it will also implicitly be marked active.
Reject Duplicate Finalizer Keys: If this finalizer key was registered before (and still exists) even by other block producers, reject the registration.

Activate Finalizer Key: Action to mark a finalizer key as active.

Registered Finalizer Keys Only: must be a registered finalizer key.
Registered BPs Only: account must be a registered block producer.
Only One Active Finalizer Key: A block producer may only have one active finalizer key.
Deactivates Previously Active Finalizer Key: Activating a finalizer key of a block producer implicitly deactivates the previously active finalizer key of that block producer.
Immediately Change Finalizer Policy: If the block producer is currently active (in top 21), then immediately call set_finalizers with the new policy after activating the finalizer key. Otherwise, wait until the periodic check that sets the proposers and finalizers.

Delete Finalizer Key: A registered block producer can delete a registered finalizer key that is not marked as active.

Prevent Deletion of Active Key: A registered block producer should not be able to delete a registered finalizer key that is marked as active unless it was the last registered finalizer key of that block producer.