Limit visibility of id method for SessionContext to the crate only (payjoin#713)

Because SessionContext itself is limited in visibility to the crate this
id method should also be limited to crate visibility.

Based on the conversation linked here,
payjoin#698 (comment)

Author: spacebear21

payjoin/src/receive/v2/mod.rs

@@ -58,7 +58,7 @@ impl SessionContext {
     }
 
     /// The per-session identifier
-    pub fn id(&self) -> ShortId {
+    pub(crate) fn id(&self) -> ShortId {
         sha256::Hash::hash(&self.s.public_key().to_compressed_bytes()).into()
     }
 }