Add new id method for UncheckedProposal

The Receiver Struct has a way of getting the short id but the UncheckedProposal
does not. This adds a similar id method for getting the short id instead for the
session context instead of exposing the whole context.

Author: benalleng

payjoin/src/receive/v2/mod.rs

@@ -339,6 +339,9 @@ impl UncheckedProposal {
             _ => Err(InternalSessionError::UnexpectedStatusCode(response.status()).into()),
         }
     }
+
+    /// The per-session identifier
+    pub fn id(&self) -> ShortId { id(&self.context.s) }
 }
 
 /// Typestate to validate that the Original PSBT has no receiver-owned inputs.