Validate ISO 4217 compliance in BOLT12 `CurrencyCode`

[tnull]

For other strings we make sure via PrintableString/UntrustableString that they are safe to print/log, etc. However, so far we omit validating the iso4217_code: CurrencyCode at all.

We should probably start validating that the supplied chars comply with the spec's alphabetic codes (i.e., are in A..Z, IIUC). At the very least we should check that they do not include escape characters, or provide a safe Display implementation (although stricter validation seams reasonable?).

[Prabhat1308]

Would it make sense to create a utility to just check from an available table of content storing the valid codes ? https://www.iban.com/currency-codes has the valid updated ISO 4217 codes which can be used .

[tnull]

Would it make sense to create a utility to just check from an available table of content storing the valid codes ? https://www.iban.com/currency-codes has the valid updated ISO 4217 codes which can be used .

Hmm, maybe, but I'm afraid that this might end up being too strict if we don't keep our representation of that list updated regularly? I think we can just check that the chars are in the alphabetical uppercase range.

[tnull]

Seems this will be closed by #3814 already.