feat: add id() method to Secp256k1PrivateKey

Feature parity with ed25519 and rsa

test/secp256k1.spec.js

@@ -85,6 +85,15 @@ describe('secp256k1 keys', () => {
     })
   })
 
+  it('key id', (done) => {
+    key.id((err, id) => {
+      expect(err).to.not.exist()
+      expect(id).to.exist()
+      expect(id).to.be.a('string')
+      done()
+    })
+  })
+
   describe('key equals', () => {
     it('equals itself', () => {
       expect(key.equals(key)).to.eql(true)