Move validation logic out of `CreateRecipients` and instead take the
already validated outputs from `ParseOutputs` as an input.
Move SFFO parsing out of `CreateRecipients` into a new function,
`InterpretSubtractFeeFromOutputsInstructions`. This takes the SFFO instructions
from `sendmany` and `sendtoaddress` and turns them into a set of integers.
In a later commit, we will also move the SFFO parsing logic from
`FundTransaction` into this function.
Worth noting: a user can pass duplicate addresses and addresses that dont exist
in the transaction outputs as SFFO args to `sendmany` and `sendtoaddress`
without triggering a warning. This behavior is preserved in to keep this commit
strictly a refactor.
5ad19668db