chore(core): isCfnResource allows any type as input (aws#28001)

It is more or less frustrating that we have to check if a variable is undefined or not before calling the `CfnResource.isCfnResource` method. For example,

```ts
const bucket1 = new Bucket(stack, 'Bucket1');
const bucket1Resource = bucket1.node.defaultChild;
if (bucket1Resource !== undefined &&  // Currently we need this!
    cdk.CfnResource.isCfnResource(bucket1Resource)
) {
    bucket1Resource.addDependency(...);
}
```

With this PR, `isCfnResource` now accepts `any` type as input and performs the necessary validations inside.

```ts
const bucket1 = new Bucket(stack, 'Bucket1');
const bucket1Resource = bucket1.node.defaultChild;
if (cdk.CfnResource.isCfnResource(bucket1Resource)) { // much smoother
    bucket1Resource.addDependency(...);
}
```

Actually, other `isXxx` methods have consistent signatures like the one below:

```ts
public static isStack(x: any): x is Stack
public static isReference(x: any): x is Reference
public static isCfnElement(x: any): x is CfnElement
// and more...
```

This change also makes the `isCfnResource` consistent with these signatures.

Note that this is not a breaking change, because the input constraint is relaxed, not tightened, so all the old code will work without change.

----

*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

packages/aws-cdk-lib/core/lib/cfn-resource.ts

@@ -4,7 +4,7 @@ import { CfnCondition } from './cfn-condition';
 /* eslint-disable import/order */
 import { CfnRefElement } from './cfn-element';
 import { CfnCreationPolicy, CfnDeletionPolicy, CfnUpdatePolicy } from './cfn-resource-policy';
-import { Construct, IConstruct, Node } from 'constructs';
+import { Construct, Node } from 'constructs';
 import { addDependency, obtainDependencies, removeDependency } from './deps';
 import { CfnReference } from './private/cfn-reference';
 import { Reference } from './reference';
@@ -34,10 +34,10 @@ export interface CfnResourceProps {
  */
 export class CfnResource extends CfnRefElement {
   /**
-   * Check whether the given construct is a CfnResource
+   * Check whether the given object is a CfnResource
    */
-  public static isCfnResource(construct: IConstruct): construct is CfnResource {
-    return (construct as any).cfnResourceType !== undefined;
+  public static isCfnResource(x: any): x is CfnResource {
+    return x !== null && typeof(x) === 'object' && x.cfnResourceType !== undefined;
   }
 
   // MAINTAINERS NOTE: this class serves as the base class for the generated L1

packages/aws-cdk-lib/core/test/cfn-resource.test.ts

@@ -412,4 +412,28 @@ describe('cfn resource', () => {
       delete process.env.CDK_DEBUG;
     }
   });
+
+  test('isCfnResource returns true with a CfnResource', () => {
+    const app = new core.App();
+    const stack = new core.Stack(app, 'Stack');
+    const res = new core.CfnResource(stack, 'SomeCfnResource', {
+      type: 'Some::Resource',
+    });
+
+    // THEN
+    expect(core.CfnResource.isCfnResource(res)).toBe(true);
+  });
+
+  test('isCfnResource returns false with a construct', () => {
+    const app = new core.App();
+    const stack = new core.Stack(app, 'Stack');
+
+    // THEN
+    expect(core.CfnResource.isCfnResource(stack)).toBe(false);
+  });
+
+  test('isCfnResource returns false with undefined', () => {
+    // THEN
+    expect(core.CfnResource.isCfnResource(undefined)).toBe(false);
+  });
 });