Introduce CurrencyConversion trait

Adds the `CurrencyConversion` trait to allow users to define custom
logic for converting fiat amounts into millisatoshis (msat).

This abstraction lays the groundwork for supporting Offers denominated
in fiat currencies, where conversion is inherently context-dependent.

Author: shaavan

lightning/src/offers/invoice_request.rs

@@ -77,8 +77,9 @@ use crate::offers::merkle::{
 };
 use crate::offers::nonce::Nonce;
 use crate::offers::offer::{
-	Amount, ExperimentalOfferTlvStream, ExperimentalOfferTlvStreamRef, Offer, OfferContents,
-	OfferId, OfferTlvStream, OfferTlvStreamRef, EXPERIMENTAL_OFFER_TYPES, OFFER_TYPES,
+	Amount, CurrencyCode, ExperimentalOfferTlvStream, ExperimentalOfferTlvStreamRef, Offer,
+	OfferContents, OfferId, OfferTlvStream, OfferTlvStreamRef, EXPERIMENTAL_OFFER_TYPES,
+	OFFER_TYPES,
 };
 use crate::offers::parse::{Bolt12ParseError, Bolt12SemanticError, ParsedMessage};
 use crate::offers::payer::{PayerContents, PayerTlvStream, PayerTlvStreamRef};
@@ -574,6 +575,34 @@ impl AsRef<TaggedHash> for UnsignedInvoiceRequest {
 	}
 }
 
+/// A trait for converting fiat currencies into millisatoshis (msats).
+///
+/// Implementations must return the conversion rate in **msats per minor unit** of the currency,
+/// where the minor unit is determined by its ISO-4217 exponent:
+/// - USD (exponent 2) → per **cent** (0.01 USD), not per dollar.
+/// - JPY (exponent 0) → per **yen**.
+/// - KWD (exponent 3) → per **fils** (0.001 KWD).
+///
+/// # Caution
+///
+/// Returning msats per major unit will be off by a factor of 10^exponent (e.g. 100× for USD).
+///
+/// This convention ensures amounts remain precise and purely integer-based when parsing and
+/// validating BOLT12 invoice requests.
+pub trait CurrencyConversion {
+	/// Converts a fiat currency specified by its ISO-4217 code into **msats per minor unit**.
+	fn fiat_to_msats(&self, iso4217_code: CurrencyCode) -> Result<u64, ()>;
+}
+
+/// A default implementation of the `CurrencyConversion` trait that does not support any currency conversions.
+pub struct DefaultCurrencyConversion;
+
+impl CurrencyConversion for DefaultCurrencyConversion {
+	fn fiat_to_msats(&self, _iso4217_code: CurrencyCode) -> Result<u64, ()> {
+		Err(())
+	}
+}
+
 /// An `InvoiceRequest` is a request for a [`Bolt12Invoice`] formulated from an [`Offer`].
 ///
 /// An offer may provide choices such as quantity, amount, chain, features, etc. An invoice request