key: add GetSecp256k1SignContext access function

This is needed in order to move the `CreateMuSig2{Nonce,PartialSig}`
functions to the musig.cpp modules, see next two commits.

src/key.cpp

@@ -568,6 +568,11 @@ bool ECC_InitSanityCheck() {
     return key.VerifyPubKey(pubkey);
 }
 
+secp256k1_context* GetSecp256k1SignContext()
+{
+    return secp256k1_context_sign;
+}
+
 /** Initialize the elliptic curve support. May not be called twice without calling ECC_Stop first. */
 static void ECC_Start() {
     assert(secp256k1_context_sign == nullptr);