Wallet: determine valid amounts from mint's keys response

[prusnak]

Same as cashubtc/nutshell#618

ISTM that CDK also hardcodes/assumes amounts of 2^n and not the ones provided by the mint:

cdk/crates/cdk/src/wallet/mod.rs

Lines 756 to 758 in f978b08

	let all_possible_amounts: Vec<usize> = (0..32).map(|i| 2usize.pow(i as u32)).collect();

[thesimplekid]

Yes I noticed this a bit ago but haven't fixed it yet.

#124