CIP-0030 getCollateral could be made more specific

[klntsky]

CIPs/CIP-0030/README.md

Line 222 in 5b50794

This shall return a list of one or more UTXOs (unspent transaction outputs) controlled by the wallet that are required to reach **AT LEAST** the combined ADA value target specified in `amount` **AND** the best suitable to be used as collateral inputs for transactions with plutus script inputs (pure ADA-only utxos). If this cannot be attained, an error message with an explanation of the blocking problem shall be returned. **NOTE:** wallets are free to return utxos that add up to a **greater** total ADA value than requested in the `amount` parameter, but wallets must never return any result where utxos would sum up to a smaller total ADA value, instead in a case like that an error message must be returned.

This shall return a list of one or more UTXOs (unspent transaction outputs) controlled by the wallet that are required to reach AT LEAST the combined ADA value target specified in amount AND the best suitable to be used as collateral inputs for transactions with plutus script inputs (pure ADA-only utxos).

The problem with these requirements is that they are too broad: the implementation that simply returns all UTxOs satisfies them.

I'd suggest enforcing the following property:

• All strict subsets of the set of returned UTxOs MUST NOT satisfy the minimum ADA collateral requirements (no unnecessary UTxOs must be returned)

Notably, one of the wallets (Eternl) breaks this property, so we cannot simply assume that UTxOs returned as collateral must not be spent, because it's possible that no more (or not enough) UTxOs are left (context)

[klntsky]

Another property that can be potentially enforced: the intersection of getUtxos() and getCollateral() sets must be empty.