feat: 🎸 getLegs can now return off-chain legs

Previous commit `e06ac80` introduced a breaking change that was not
marked. `Instruction.getLegs()` can now return off-chain legs.

Legs can be filtered in a type safe manner using `isOffChainLeg` to
be compatible with prior behavior.  e.g.

`legs.filter((leg): leg is Exclude<Leg, OffChainLeg> => !isOffChainLeg(leg))`

`AffirmOrWithdrawInstructionParams` was renamed to `WithdrawInstructionParams`

BREAKING CHANGE: 🧨 getLegs may return off-chain legs