You should refer to API reference document to get a clear picture. There are examples for such use cases.

However, since you have already asked here and my hands has been itchy to provide you with an answer, so here goes the TypeScript implementation of just the IAM part:

import { 
   ManagedPolicy, 
   Role, 
   ServicePrincipal, 
   PolicyStatement, 
   Effect 
} from '@aws-cdk/aws-iam';

....
....

const ecsFargateServiceRole = new Role(this, 'FargateTaskExecutionServiceRole', {
  assumedBy: new ServicePrincipal('ecs-tasks.amazonaws.com')
});

// Add a policy to a Role
ecsFargateServiceRole.addToPolicy(
  new PolicyStatement({
    effect: Effect.ALLOW,
    resources: ['*'],
    actions: [            
      'ecr:GetAuthorizationToken',
      'ecr:BatchCheckLayerAvailability',
      'ecr:GetDownloadUrlForLayer',
      'ecr:BatchGetImage',
      'logs:CreateLogStream',
      'logs:PutLogEvents'
    ]
  })
);

// Add a managed policy to a role you can use
ecsFargateServiceRole.addManagedPolicy(
    ManagedPolicy.fromAwsManagedPolicyName('AmazonECSTaskExecutionRolePolicy')
);

....
....

UPDATE:

When you are adding an AWS managed policy to a role, you can get the managed policy as a reference by its name or by its ARN. The important part is that if an AWS Managed policy is used as above by its name or ARN, then you will not need to use the policy statement explicitly. From my answer above, you can use the managed policy approach rather than using the policy statement.

An easy way to define the role now would be:

const ecsFargateServiceRole = new Role(this, 'FargateTaskExecutionServiceRole', {
  assumedBy: new ServicePrincipal('ecs-tasks.amazonaws.com'),
  managedPolicies: [
    ManagedPolicy.fromAwsManagedPolicyName('AmazonECSTaskExecutionRolePolicy')
  ]
});

Note that I have excluded the constructor for the Construct for brevity.