refactor: Define PaymentTransaction model (#1542)

- Defines a TypeScript type for PaymentTransaction
- Provides an optional function to users for verifying a PaymentTransaction instance at runtime: verifyPaymentTransaction()
- Adds tests for verifyPaymentTransaction()
- Adds isFlagEnabled() util to be used for models

test/models/utils.ts

@@ -0,0 +1,29 @@
+import { isFlagEnabled } from '../../src/models/utils'
+import { assert } from 'chai'
+
+/**
+ * Utils Testing
+ *
+ * Provides tests for utils used in models
+ */
+describe('Models Utils', () => {
+    describe('isFlagEnabled', () => {
+        let flags
+        const flag1 = 0x00010000
+        const flag2 = 0x00020000
+
+        beforeEach(() => {
+            flags = 0x00000000
+        })
+
+        it('verifies a flag is enabled', () => {
+            flags += flag1 + flag2
+            assert.isTrue(isFlagEnabled(flags, flag1))
+        })
+
+        it('verifies a flag is not enabled', () => {
+            flags += flag2
+            assert.isFalse(isFlagEnabled(flags, flag1))
+        })
+    })
+})