Add a method to get the Certificate type to the interface

[wolf31o2]

A common use case for Certificate data will be to only operate on specific certificate types. It would greatly simplify downstream if we had a way to access the type from the Certificate interface.

[agaffney]

The idea is to use Go's type switch/assertion for determining the cert type, since multiple types are returned using the ledger.Certificate interface. However, we have the information readily available to also return the integer value for the cert type, so adding a .Type() function wouldn't be difficult.

Type switch example:

switch v := myCert.(type) {
case ledger.StakeDelegationCertificate:
    // you can access v.StakeCredential and v.PoolKeyHash directly here
case ledger.PoolRegistrationCertificate:
    // you can access v.Operator, v.Pledge, v.Margin, etc. directly here
}