Add Display impl for DateTime (#3235)

## Motivation and Context
<!--- Why is this change required? What problem does it solve? -->
It implements Display trait For DateTime

<!--- If it fixes an open issue, please link to the issue here -->
https://github.com/smithy-lang/smithy-rs/issues/3161



## Description
<!--- Describe your changes in detail -->
I implemented Display trait for DateTime

## Testing
<!--- Please describe in detail how you tested your changes -->
<!--- Include details of your testing environment, and the tests you ran
to -->

I used this test
```rust
    fn test_display_formatting() {
        // Create a DateTime instance for testing
        let datetime = DateTime {
            seconds: 1636761600, // Example timestamp (replace with your actual timestamp)
            subsecond_nanos: 123456789, // Example subsecond nanos (replace with your actual value)
        };

        // Expected RFC-3339 formatted string
        let expected = "2021-11-13T00:00:00.123456789Z";

        // Format the DateTime using Display trait
        let formatted = format!("{}", datetime);

        // Assert that the formatted string matches the expected result
        assert_eq!(formatted, expected);
    }
```

<!--- see how your change affects other areas of the code, etc. -->

## Checklist
<!--- If a checkbox below is not applicable, then please DELETE it
rather than leaving it unchecked -->
- [ ] I have updated `CHANGELOG.next.toml` if I made changes to the
smithy-rs codegen or runtime crates
- [ ] I have updated `CHANGELOG.next.toml` if I made changes to the AWS
SDK, generated SDK code, or SDK runtime crates

----

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choice._

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Co-authored-by: Hakan Vardar <hakovardo123@gmail.com>