fix(appsync): source api association does not depend on schema (#29455)

### Issue # (if applicable)

Closes #29044.

### Reason for this change

When associating between the source GraphQL API and the merged API using `fromSourceApis`, generated association resource doesn't depend on the source GraphQL schema.

```ts
// This API has `CfnGraphQLSchema` by `definition` prop
const firstApi = new appsync.GraphqlApi(stack, 'FirstSourceAPI', {
  name: 'FirstSourceAPI',
  definition: appsync.Definition.fromFile(path.join(__dirname, 'appsync.merged-api-1.graphql')),
});

// This merged API generates `CfnSourceApiAssociation`
new appsync.GraphqlApi(stack, 'MergedAPI', {
  name: 'MergedAPI',
  definition: appsync.Definition.fromSourceApis({
    sourceApis: [
      {
        sourceApi: firstApi,
        mergeType: appsync.MergeType.MANUAL_MERGE,
      },
    ],
  }),
});
```

The same is true if the `SourceApiAssociation` construct is used explicitly.

```ts
// This API has `CfnGraphQLSchema` by `definition` prop
const firstApi = new appsync.GraphqlApi(stack, 'FirstSourceAPI', {
  name: 'FirstSourceAPI',
  definition: appsync.Definition.fromFile(path.join(__dirname, 'appsync.merged-api-1.graphql')),
});

// This merged API does not generate `CfnSourceApiAssociation`
const mergedApi = new appsync.GraphqlApi(stack, 'MergedAPI', {
  name: 'MergedAPI',
  definition: appsync.Definition.fromSourceApis({
    sourceApis: [],
    mergedApiExecutionRole: mergedApiExecutionRole,
  }),
});

// This construct has `CfnSourceApiAssociation`
new appsync.SourceApiAssociation(stack, 'SourceApiAssociation1', {
  sourceApi: firstApi,
  mergedApi: mergedApi,
  mergeType: appsync.MergeType.MANUAL_MERGE,
  mergedApiExecutionRole: mergedApiExecutionRole,
});
```

### Description of changes

The `sourceApi` passed by the `fromSourceApis` method or the `SourceApiAssociation` construct has `addSchemaDependency` method. Using this method, we can make the association to depend on the schema in the `sourceApi`.
But, if the api is an IMPORTED resource to begin with, it has no schema in the CDK layer, so there is nothing we can do about it. (The method does nothing.)

### Description of how you validated changes

Both unit and existing integ tests.

### Checklist
- [x] My code adheres to the [CONTRIBUTING GUIDE](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md) and [DESIGN GUIDELINES](https://github.com/aws/aws-cdk/blob/main/docs/DESIGN_GUIDELINES.md)

----

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

packages/@aws-cdk-testing/framework-integ/test/aws-appsync/test/integ.appsync-merged-api-arn-flag-enabled.js.snapshot/stack.template.json

@@ -213,7 +213,10 @@
       "Arn"
      ]
     }
-   }
+   },
+   "DependsOn": [
+    "FirstSourceAPISchemaF2FDB692"
+   ]
   },
   "MergedAPISecondSourceAPIAssociationBD1A08F4": {
    "Type": "AWS::AppSync::SourceApiAssociation",
@@ -233,7 +236,10 @@
       "Arn"
      ]
     }
-   }
+   },
+   "DependsOn": [
+    "SecondSourceAPISchema65B7401E"
+   ]
   },
   "MergedAPIDefaultApiKeyAF5EA13C": {
    "Type": "AWS::AppSync::ApiKey",