feat(appsync): add the grant method to the imported GraphqlApi (#29086)

### Issue

Closes #23031.

### Reason for this change

The `grantXx` methods are implemented in the `GraphqlApi` class, but it could not be used with an imported graphql api.

### Description of changes

Moved the implementation of the `grantXx` method from the `GraphqlApi` class to the `GraphqlApiBase` class."

### Description of how you validated changes

I've added both unit and 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*

Author: badmintoncryer

packages/@aws-cdk-testing/framework-integ/test/aws-appsync/test/integ.graphql-grant-imported-api.js.snapshot/GraphqlGrantImportedApiIntegDefaultTestDeployAssertE072C285.assets.json

@@ -0,0 +1,116 @@
+{
+ "Resources": {
+  "PoolD3F588B8": {
+   "Type": "AWS::Cognito::UserPool",
+   "Properties": {
+    "AccountRecoverySetting": {
+     "RecoveryMechanisms": [
+      {
+       "Name": "verified_phone_number",
+       "Priority": 1
+      },
+      {
+       "Name": "verified_email",
+       "Priority": 2
+      }
+     ]
+    },
+    "AdminCreateUserConfig": {
+     "AllowAdminCreateUserOnly": true
+    },
+    "EmailVerificationMessage": "The verification code to your new account is {####}",
+    "EmailVerificationSubject": "Verify your new account",
+    "SmsVerificationMessage": "The verification code to your new account is {####}",
+    "UserPoolName": "myPool",
+    "VerificationMessageTemplate": {
+     "DefaultEmailOption": "CONFIRM_WITH_CODE",
+     "EmailMessage": "The verification code to your new account is {####}",
+     "EmailSubject": "Verify your new account",
+     "SmsMessage": "The verification code to your new account is {####}"
+    }
+   },
+   "UpdateReplacePolicy": "Delete",
+   "DeletionPolicy": "Delete"
+  },
+  "ApiF70053CD": {
+   "Type": "AWS::AppSync::GraphQLApi",
+   "Properties": {
+    "AdditionalAuthenticationProviders": [
+     {
+      "AuthenticationType": "AWS_IAM"
+     }
+    ],
+    "AuthenticationType": "AMAZON_COGNITO_USER_POOLS",
+    "Name": "Integ_Test_IAM",
+    "UserPoolConfig": {
+     "AwsRegion": {
+      "Ref": "AWS::Region"
+     },
+     "DefaultAction": "ALLOW",
+     "UserPoolId": {
+      "Ref": "PoolD3F588B8"
+     }
+    }
+   }
+  },
+  "ApiSchema510EECD7": {
+   "Type": "AWS::AppSync::GraphQLSchema",
+   "Properties": {
+    "ApiId": {
+     "Fn::GetAtt": [
+      "ApiF70053CD",
+      "ApiId"
+     ]
+    },
+    "Definition": "type test @aws_iam {\n  id: String!\n  version: String!\n}\n\ntype Query {\n  getTest(id: String!): test\n  getTests: [ test! ]\n    @aws_iam \n}\n\ninput TestInput {\n  version: String!\n}\n\ntype Mutation {\n  addTest(input: TestInput!): test\n    @aws_iam\n}\n"
+   }
+  }
+ },
+ "Outputs": {
+  "ExportsOutputFnGetAttApiF70053CDApiIdF185726B": {
+   "Value": {
+    "Fn::GetAtt": [
+     "ApiF70053CD",
+     "ApiId"
+    ]
+   },
+   "Export": {
+    "Name": "aws-appsync-integ:ExportsOutputFnGetAttApiF70053CDApiIdF185726B"
+   }
+  }
+ },
+ "Parameters": {
+  "BootstrapVersion": {
+   "Type": "AWS::SSM::Parameter::Value<String>",
+   "Default": "/cdk-bootstrap/hnb659fds/version",
+   "Description": "Version of the CDK Bootstrap resources in this environment, automatically retrieved from SSM Parameter Store. [cdk:skip]"
+  }
+ },
+ "Rules": {
+  "CheckBootstrapVersion": {
+   "Assertions": [
+    {
+     "Assert": {
+      "Fn::Not": [
+       {
+        "Fn::Contains": [
+         [
+          "1",
+          "2",
+          "3",
+          "4",
+          "5"
+         ],
+         {
+          "Ref": "BootstrapVersion"
+         }
+        ]
+       }
+      ]
+     },
+     "AssertDescription": "CDK bootstrap stack version 6 required. Please run 'cdk bootstrap' with a recent version of the CDK CLI."
+    }
+   ]
+  }
+ }
+}