common: calculate correct weight of Taproot spends

[whitslack]

utxo_spend_weight(…) was assuming that witnesses always include a pubkey, but that is not true of Taproot (key-path) spends. The effect was that wallet-funded commands (e.g., withdraw, fundchannel, multifundchannel, etc.) were overpaying on fees due to calculating Taproot input weights as 34 sipa greater than they actually are.

Also as a consequence of the miscalculation, users who carefully select a set of UTxOs to spend so as to avoid producing a change output would experience an error when trying to spend them. As an example, I tried to spend a tiny 1390-sat P2TR output to a new 1275-sat output at minimum feerate. The actual weight of the transaction is 451 sipa, so the 115-sat fee yields a feerate of 255 sat/kw. However, CLN refuses the transaction, claiming:

{
   "code": 301,
   "message": "Could not afford 1275sat using UTXOs totalling 1390sat with weight 485 at feerate 253"
}

The reported weight of 485 sipa is wrong, as it assumes the input will include a 34-sipa public key when in fact it will not.

This commit amends the bitcoin_tx_simple_input_witness_weight() and bitcoin_tx_simple_input_weight(…) functions to take a parameter specifying the scriptPubKey type of the spend. The implementation of the former is corrected so that P2TR spends are not charged for a public key that they actually lack.

An enum type is introduced to enumerate the known scriptPubKey types, and a new scriptpubkey_type(…) utility function is implemented to discern the type of a given scriptPubKey. The existing is_known_scripttype(…) function is reimplemented as an inline wrapper around the new function.

The default_lease_rates(…) function in plugins/funder_policy.c is amended so as to state explicitly an assumption that it has been making: that the two inputs it assumes for its default max weight calculation will not be Taproot inputs.

Fixes: #8164
Changelog-Fixed: P2TR inputs are assessed the correct weight in fee calculations.

Checklist

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