Add useful Debug impl to PropertyBag and ConfigBag (#2612)

## Motivation and Context
- it's hard to debug what properties are set, especially for layered
configuration

## Description
Add Debug Info for PropertyBag and Config bag:
```
PropertyBag { contents: ["aws_smithy_http::property_bag::test::test_extensions::MyType"] }
```

```

ConfigBag {
    layers: [
        Layer {
            name: "c",
            properties: [
                "aws_smithy_runtime_api::config_bag::Value<aws_smithy_runtime_api::config_bag::test::layered_property_bag::Prop3>",
                "aws_smithy_runtime_api::config_bag::Value<aws_smithy_runtime_api::config_bag::test::layered_property_bag::Prop4>",
            ],
        },
        Layer {
            name: "b",
            properties: [
                "aws_smithy_runtime_api::config_bag::Value<aws_smithy_runtime_api::config_bag::test::layered_property_bag::Prop3>",
                "aws_smithy_runtime_api::config_bag::Value<aws_smithy_runtime_api::config_bag::test::layered_property_bag::Prop2>",
            ],
        },
        Layer {
            name: "a",
            properties: [
                "aws_smithy_runtime_api::config_bag::Value<aws_smithy_runtime_api::config_bag::test::layered_property_bag::Prop1>",
            ],
        },
        Layer {
            name: "base",
            properties: [],
        },
    ],
}
```

There is still some work to do, but this is a start

## Testing
- [x] UT

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

----

_By submitting this pull request, I confirm that you can use, modify,
copy, and redistribute this contribution, under the terms of your
choice._

---------

Co-authored-by: John DiSanti <jdisanti@amazon.com>