Expose de/serialize for Sender, Receiver

These structs need to be serialized and deserialized in order to be
persisted in a downstream database.

We use json to do this only because that decision was made in payjoin-cli.
A binary encoding could be used as well, but that bikeshed is to be built
at a later date.

Cargo.toml

@@ -26,6 +26,7 @@ bitcoin-ffi = { git = "https://github.com/bitcoindevkit/bitcoin-ffi.git", rev =
 hex = "0.4.3"
 ohttp = { package = "bitcoin-ohttp", version = "0.6.0" }
 payjoin = { version = "0.21.0", features = ["send", "receive", "base64", "v2", "io"] }
+serde_json = "1.0.128"
 thiserror = "1.0.58"
 uniffi = { version = "0.28.0", optional = true }
 url = "2.5.0"

src/error.rs

@@ -74,6 +74,9 @@ pub enum PayjoinError {
 
     #[error("{message}")]
     InputPairError { message: String },
+
+    #[error("{message}")]
+    SerdeJsonError { message: String },
 }
 
 macro_rules! impl_from_error {
@@ -99,6 +102,7 @@ impl_from_error! {
     OutputSubstitutionError => OutputSubstitutionError,
     InputContributionError => InputContributionError,
     PsbtInputError => InputPairError,
+    serde_json::Error => SerdeJsonError,
 }
 
 #[cfg(feature = "uniffi")]

src/receive/mod.rs

@@ -97,6 +97,16 @@ impl Receiver {
     pub fn id(&self) -> String {
         <Self as Into<payjoin::receive::v2::Receiver>>::into(self.clone()).id().to_string()
     }
+
+    pub fn to_json(&self) -> Result<String, PayjoinError> {
+        serde_json::to_string(&self.0).map_err(|e| e.into())
+    }
+
+    pub fn from_json(&self, json: &str) -> Result<Self, PayjoinError> {
+        let receiver = serde_json::from_str::<payjoin::receive::v2::Receiver>(json)
+            .map_err(<serde_json::Error as Into<PayjoinError>>::into)?;
+        Ok(receiver.into())
+    }
 }
 
 #[derive(Clone)]

src/receive/uni.rs

@@ -87,6 +87,14 @@ impl Receiver {
     pub fn id(&self) -> String {
         self.0.id()
     }
+
+    pub fn to_json(&self) -> Result<String, PayjoinError> {
+        self.0.to_json()
+    }
+
+    pub fn from_json(&self, json: &str) -> Result<Self, PayjoinError> {
+        self.0.from_json(json).map(Into::into)
+    }
 }
 
 #[cfg_attr(feature = "uniffi", derive(uniffi::Record))]

src/send/mod.rs

@@ -124,6 +124,16 @@ impl Sender {
             Err(e) => Err(e.into()),
         }
     }
+
+    pub fn to_json(&self) -> Result<String, PayjoinError> {
+        serde_json::to_string(&self.0).map_err(|e| e.into())
+    }
+
+    pub fn from_json(&self, json: &str) -> Result<Self, PayjoinError> {
+        let sender = serde_json::from_str::<payjoin::send::Sender>(json)
+            .map_err(<serde_json::Error as Into<PayjoinError>>::into)?;
+        Ok(sender.into())
+    }
 }
 
 /// Data required for validation of response.

src/send/uni.rs

@@ -125,6 +125,14 @@ impl Sender {
             Err(e) => Err(e),
         }
     }
+
+    pub fn to_json(&self) -> Result<String, PayjoinError> {
+        self.0.to_json()
+    }
+
+    pub fn from_json(&self, json: &str) -> Result<Self, PayjoinError> {
+        self.0.from_json(json).map(Into::into)
+    }
 }
 
 #[derive(uniffi::Object)]