Introduce recurrence fields in Offer (BOLT12 PoC)

This commit begins the introduction of BOLT12 recurrence support in LDK.

It adds the core recurrence-related fields to `Offer`, enabling
subscription-style and periodic payments as described in the draft spec.
Since this is a PoC, the focus is on establishing the data model and
documenting the intended semantics. Where the spec is ambiguous or
redundant, accompanying comments note possible simplifications or
improvements.

This lays the foundation for the following commits, which will implement
invoice-request parsing, payee-side validation, and period/paywindow
handling.

Spec reference:
https://github.com/rustyrussell/bolts/blob/guilt/offers-recurrence/12-offer-encoding.md#tlv-fields-for-offers

Author: shaavan

lightning/src/offers/invoice.rs

@@ -1987,6 +1987,11 @@ mod tests {
 					issuer: None,
 					quantity_max: None,
 					issuer_id: Some(&recipient_pubkey()),
+					recurrence_compulsory: None,
+					recurrence_optional: None,
+					recurrence_base: None,
+					recurrence_paywindow: None,
+					recurrence_limit: None,
 				},
 				InvoiceRequestTlvStreamRef {
 					chain: None,
@@ -2090,6 +2095,11 @@ mod tests {
 					issuer: None,
 					quantity_max: None,
 					issuer_id: None,
+					recurrence_compulsory: None,
+					recurrence_optional: None,
+					recurrence_base: None,
+					recurrence_paywindow: None,
+					recurrence_limit: None,
 				},
 				InvoiceRequestTlvStreamRef {
 					chain: None,

lightning/src/offers/invoice_request.rs

@@ -1647,6 +1647,11 @@ mod tests {
 					issuer: None,
 					quantity_max: None,
 					issuer_id: Some(&recipient_pubkey()),
+					recurrence_compulsory: None,
+					recurrence_optional: None,
+					recurrence_base: None,
+					recurrence_paywindow: None,
+					recurrence_limit: None,
 				},
 				InvoiceRequestTlvStreamRef {
 					chain: None,