Comparable Arns

### Describe the feature

It'd be nice if [Arn](https://github.com/aws/aws-sdk-java-v2/blob/master/core/arns/src/main/java/software/amazon/awssdk/arns/Arn.java) implemented `Comparable`.

This would make Arns usable in sorted collection types, etc.

### Is your Feature Request related to a problem?

Not really.
- We can use `HashMap` instead of `TreeMap`, etc.
- We can sort Arns by converting to strings, sorting, then converting back to Arns.

So this would just make Arns more flexible.

### Proposed Solution

Here is a straightforward implementation:

public static final Comparator COMPARATOR =
Comparator.comparing(Arn::toString);

@Override
public int compareTo(Arn rhs) {
return COMPARATOR.compare(this, rhs);
}

Here is a more elaborate one, which may be more efficient by avoiding the String construction:

public static final Comparator COMPARATOR =
Comparator.comparing(Arn::partition)
.thenComparing(Arn::service)
.thenComparing(x -> x.region, Comparator.nullsLast(Comparator.naturalOrder()))
.thenComparing(x -> x.accountId, Comparator.nullsLast(Comparator.naturalOrder()))
.thenComparing(Arn::resourceAsString);

@Override
public int compareTo(Arn rhs) {
return COMPARATOR.compare(this, rhs);
}

### Describe alternatives you've considered

_No response_

### Acknowledge

- [X] I may be able to implement this feature request

### AWS Java SDK version used

aws-sdk-java-v2 c0eb030fceaa9ccb64ad8acd74445f4e5ee3a547

### JDK version used

openjdk version "1.8.0_312"

### Operating System and version

MacOS Big Sur Version 11.6.2 (20G314)