use getSharedSecret

Use getSharedSecret instead of multiply directly. Did some digging and getSharedSecret
is using multiply under the hood, but looks like there is an outstanding todo to make
it constant time, so if/when that happens we wont need to update anything here.

src/index.ts

@@ -84,7 +84,7 @@ export class SilentPayment {
     for (const group of silentPaymentGroups) {
       // Bscan * a * outpoint_hash
       const ecdh_shared_secret_step1 = Buffer.from(ecc.privateMultiply(outpoint_hash, a) as Uint8Array);
-      const ecdh_shared_secret = ecc.pointMultiply(group.Bscan, ecdh_shared_secret_step1);
+      const ecdh_shared_secret = Buffer.from(ecc.getSharedSecret(ecdh_shared_secret_step1, group.Bscan) as Uint8Array);
 
       let k = 0;
       for (const [Bm, amount] of group.BmValues) {

src/noble_ecc.ts

@@ -58,6 +58,10 @@ const ecc = {
       return { parity, xOnlyPubkey: P.slice(1) };
     }),
 
+  getSharedSecret: (sk: Uint8Array, pk: Uint8Array, compressed?: boolean): Uint8Array => {
+    return necc.getSharedSecret(sk, pk, defaultTrue(compressed));
+  },
+
   pointFromScalar: (sk: Uint8Array, compressed?: boolean): Uint8Array | null => throwToNull(() => necc.getPublicKey(sk, defaultTrue(compressed))),
 
   pointCompress: (p: Uint8Array, compressed?: boolean): Uint8Array => {