This effectively splits the `ProvisionalProposal` state into three
states that must be completed in order:
- `WantsOutputs` allows the receiver to substitute or add output(s) and
produces a WantsInputs
- `WantsInputs` allows the receiver to contribute input(s) as needed to
fund their outputs and produces a ProvisionalProposal
- `ProvisionalProposal` is only responsible for finalizing the payjoin
proposal and producing a PSBT that will be acceptable to the sender

`contribute_witness_input` is now `contribute_witness_inputs`.

It takes an arbitrary number of inputs as a HashMap and returns a
`ProvisionalProposal`.

When disable_output_substitition is true, the receiver may not change
the script pubkey or decrease the value of their original output, but is
allowed to increase the amount or add new outputs.

- Output substitution functions return a modified WantsOutputs instead
  of a WantsInputs. `freeze_outputs` is used to proceed to the
  WantsInputs step.
- Similarly, contribute_witness_inputs returns a modified WantsInputs
  and `freeze_inputs` returns the ProvisionalProposal
- One change output (aka drain script) must be identified if outputs are
  replaced. The change output receives all excess input amount from the
  receiver, and will be responsible for paying receiver fees if
  applicable.

Modify `apply_fee` to enable fee contributions from the receiver for
fees not covered by the sender. This includes
- any fees that pay for additional inputs, in excess of the sender's
  max_additional_fee_contribution.
- all fees that pay for additional *outputs*.
- in the case where the sender doesn't have a fee output (e.g. a sweep
  transaction), all tx fees are deducted from the receiver output.

Adds two integration tests for transaction cut-through:
- Receiver UTXO consolidation
- Receiver forwarding payment to a third-party

Also fixes some issues in outputs and inputs contribution logic that
became apparent while testing.

Partially addresses payjoin#312

Additional outputs should be inserted at random indices for privacy,
while preserving the relative ordering of receiver/sender outputs from
the original PSBT.

This commit also ensures that `disable_output_substitution` checks are
made for every receiver output, in the unlikely case that there are
multiple receiver outputs in the original PSBT.

This ensures that the receiver does not pay more in fees than they would
by building a separate transaction at max_feerate instead. That prevents
a scenario where the sender specifies a `minfeerate` param that would
require the receiver to pay more than they are willing to spend in fees.

Privacy-conscious receivers should err on the side of indulgence with
their preferred max feerate in order to satisfy their preference for
privacy.

This commit also adds a `max-feerate` receive option to payjoin-cli.