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);

src/key.h

@@ -16,6 +16,8 @@
 #include <stdexcept>
 #include <vector>
 
+struct secp256k1_context_struct;
+typedef struct secp256k1_context_struct secp256k1_context;
 
 /**
  * CPrivKey is a serialized private key, with all parameters included
@@ -315,6 +317,9 @@ private:
 /** Check that required EC support is available at runtime. */
 bool ECC_InitSanityCheck();
 
+/** Access the secp256k1 context used for signing and MuSig2 nonce generation. */
+secp256k1_context* GetSecp256k1SignContext();
+
 /**
  * RAII class initializing and deinitializing global state for elliptic curve support.
  * Only one instance may be initialized at a time.