musig: add pubkey_get to obtain a full pubkey from a keyagg_cache

include/secp256k1_musig.h

@@ -223,6 +223,24 @@ SECP256K1_API int secp256k1_musig_pubkey_agg(
     size_t n_pubkeys
 ) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(5);
 
+/** Obtain the aggregate public key from a keyagg_cache.
+ *
+ *  This is only useful if you need the non-xonly public key, in particular for
+ *  ordinary (non-xonly) tweaking or batch-verifying multiple key aggregations
+ *  (not implemented).
+ *
+ *  Returns: 0 if the arguments are invalid, 1 otherwise
+ *  Args:        ctx: pointer to a context object
+ *  Out:      agg_pk: the MuSig-aggregated public key.
+ *  In: keyagg_cache: pointer to a `musig_keyagg_cache` struct initialized by
+ *                    `musig_pubkey_agg`
+ */
+SECP256K1_API SECP256K1_WARN_UNUSED_RESULT int secp256k1_musig_pubkey_get(
+    const secp256k1_context* ctx,
+    secp256k1_pubkey *agg_pk,
+    secp256k1_musig_keyagg_cache *keyagg_cache
+) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2) SECP256K1_ARG_NONNULL(3);
+
 /** Tweak an x-only public key in a given keyagg_cache by adding
  *  the generator multiplied with `tweak32` to it.
  *