Add ElligatorSwift key creation and ECDH logic

Co-authored-by: Dhruv Mehta <856960+dhruv@users.noreply.github.com>

src/pubkey.h

@@ -291,6 +291,28 @@ public:
     SERIALIZE_METHODS(XOnlyPubKey, obj) { READWRITE(obj.m_keydata); }
 };
 
+/** An ElligatorSwift-encoded public key. */
+struct EllSwiftPubKey
+{
+private:
+    static constexpr size_t SIZE = 64;
+    std::array<std::byte, SIZE> m_pubkey;
+
+public:
+    /** Construct a new ellswift public key from a given serialization. */
+    EllSwiftPubKey(const std::array<std::byte, SIZE>& ellswift) :
+        m_pubkey(ellswift) {}
+
+    /** Decode to normal compressed CPubKey (for debugging purposes). */
+    CPubKey Decode() const;
+
+    // Read-only access for serialization.
+    const std::byte* data() const { return m_pubkey.data(); }
+    static constexpr size_t size() { return SIZE; }
+    auto begin() const { return m_pubkey.cbegin(); }
+    auto end() const { return m_pubkey.cend(); }
+};
+
 struct CExtPubKey {
     unsigned char version[4];
     unsigned char nDepth;