Add more NativeTokensBuilder helpers (#1353)

* Add more NativeTokensBuilder helpers

* Add Debug derivation

* Add insert method

bee-message/src/output/native_token.rs

@@ -55,7 +55,7 @@ fn verify_amount<const VERIFY: bool>(amount: &U256) -> Result<(), Error> {
 }
 
 /// A builder for [`NativeTokens`].
-#[derive(Default)]
+#[derive(Clone, Default, Debug)]
 #[must_use]
 pub struct NativeTokensBuilder(HashMap<TokenId, U256>);
 
@@ -85,6 +85,30 @@ impl NativeTokensBuilder {
         Ok(())
     }
 
+    /// Merges another [`NativeTokensBuilder`] into this one.
+    pub fn merge(&mut self, other: NativeTokensBuilder) -> Result<(), Error> {
+        for (token_id, amount) in other.0.into_iter() {
+            self.add_native_token(NativeToken::new(token_id, amount)?)?;
+        }
+
+        Ok(())
+    }
+
+    /// Returns whether the [`NativeTokensBuilder`] is empty or not.
+    pub fn is_empty(&self) -> bool {
+        self.0.is_empty()
+    }
+
+    /// Gets an amount from the [`NativeTokensBuilder`].
+    pub fn get(&self, token_id: &TokenId) -> Option<&U256> {
+        self.0.get(token_id)
+    }
+
+    /// Inserts a new entry in the [`NativeTokensBuilder`].
+    pub fn insert(&mut self, token_id: TokenId, amount: U256) -> Option<U256> {
+        self.0.insert(token_id, amount)
+    }
+
     /// Finishes the [`NativeTokensBuilder`] into [`NativeTokens`].
     pub fn finish(self) -> Result<NativeTokens, Error> {
         NativeTokens::try_from(