feat(ec2): support for the credit configuration mode for burstable in…

…stances (#28728)

In this PR, I have enabled the setting of the [credit configuration mode](https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/burstable-credits-baseline-concepts.html#key-concepts) for EC2 instances and NAT instances in burstable performance instances.

```ts
// for EC2 instance
new Instance(stack, 'Instance', {
  vpc,
  machineImage: new AmazonLinuxImage(),
  instanceType: InstanceType.of(InstanceClass.T3, InstanceSize.LARGE),
  creditSpecification: CpuCredits.STANDARD, // added
});

// for NAT instance
const natInstanceProvider = NatProvider.instance({
  instanceType: InstanceType.of(InstanceClass.T3, InstanceSize.LARGE),
  machineImage: new AmazonLinuxImage(),
  creditSpecification: CpuCredits.STANDARD, // added
});
new Vpc(stack, 'VPC', {
  natGatewayProvider: natInstanceProvider,
});
```

Closes #19166.

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