Add shorthand API derive_public_key_at_index

The name is deliberately long and not `derive_public_key` because
we already have `DefiniteDescriptorKey::derive_public_key`
Bikeshedding of names welcome

Author: sanket1729

src/descriptor/key.rs

@@ -448,6 +448,18 @@ impl DescriptorPublicKey {
         self.at_derivation_index(index)
     }
 
+    /// Derive the key at the given index.
+    /// Shorthand for `at_derivation_index(idx).derive_public_key(secp)`.
+    ///
+    /// If the key does not have a wildcard, the user can supply a dummy derivation index.
+    pub fn derive_public_key_at_index<C: Verification>(
+        self,
+        secp: &Secp256k1<C>,
+        index: u32,
+    ) -> Result<bitcoin::PublicKey, ConversionError> {
+        self.at_derivation_index(index).derive_public_key(secp)
+    }
+
     /// Replaces any wildcard (i.e. `/*`) in the key with a particular derivation index, turning it into a
     /// *definite* key (i.e. one where all the derivation paths are set).
     ///