key: Add constructor for CExtKey that takes CExtPubKey and CKey

We often need to construct a CExtKey given an CExtPubKey and CKey, so
implement a constructor that does that for us.

src/key.h

@@ -223,6 +223,12 @@ struct CExtKey {
             a.key == b.key;
     }
 
+    CExtKey() = default;
+    CExtKey(const CExtPubKey& xpub, const CKey& key_in) : nDepth(xpub.nDepth), nChild(xpub.nChild), chaincode(xpub.chaincode), key(key_in)
+    {
+        std::copy(xpub.vchFingerprint, xpub.vchFingerprint + sizeof(xpub.vchFingerprint), vchFingerprint);
+    }
+
     void Encode(unsigned char code[BIP32_EXTKEY_SIZE]) const;
     void Decode(const unsigned char code[BIP32_EXTKEY_SIZE]);
     [[nodiscard]] bool Derive(CExtKey& out, unsigned int nChild) const;