signer: Improve the E2E verification

There are a number of places where we can strengthen the existing E2E verification, or fine-tune to allow a more complete representation of the RPC semantics, e.g., by adding additional call parameters to be extracted and reflected in approvals.

- [ ] @devrandom points out that we can validate the amount and shutdown script for `ReadyChannel` (https://github.com/Blockstream/greenlight/pull/249#discussion_r1314229879)
- [ ] We need to add the `maxfeepercent` to the `pay` method to adjust the fees that we are willing to pay.
- [x] @devrandom points out that there may be overpayments for invoices, so we should extract the actual amount from the `preapproveinvoice` message (https://github.com/Blockstream/greenlight/pull/249#discussion_r1314230493)