Add MuSig2 Keyagg Cache helper functions

secp256k1 provides us secp256k1_musig_keyagg_cache objects which we are
used as part of session info and to get the aggregate pubkey. These
helper functions help us convert to/from the secp256k1 C objects into
the Bitcoin Core C++ objects.

src/musig.h

@@ -0,0 +1,22 @@
+// Copyright (c) 2024-present The Bitcoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or https://www.opensource.org/licenses/mit-license.php.
+
+#ifndef BITCOIN_MUSIG_H
+#define BITCOIN_MUSIG_H
+
+#include <pubkey.h>
+
+#include <optional>
+#include <vector>
+
+struct secp256k1_musig_keyagg_cache;
+
+//! Create a secp256k1_musig_keyagg_cache from the pubkeys in their current order. This is necessary for most MuSig2 operations
+bool GetMuSig2KeyAggCache(const std::vector<CPubKey>& pubkeys, secp256k1_musig_keyagg_cache& keyagg_cache);
+//! Retrieve the full aggregate pubkey from the secp256k1_musig_keyagg_cache
+std::optional<CPubKey> GetCPubKeyFromMuSig2KeyAggCache(secp256k1_musig_keyagg_cache& cache);
+//! Compute the full aggregate pubkey from the given participant pubkeys in their current order
+std::optional<CPubKey> MuSig2AggregatePubkeys(const std::vector<CPubKey>& pubkeys);
+
+#endif // BITCOIN_MUSIG_H