Expose the `CustomizableSend` type in each SDK trait to allow treating operations generically

### Describe the feature

I would like to expose the [`CustomizableSend`](https://docs.rs/aws-sdk-dynamodb/latest/src/aws_sdk_dynamodb/client/customize.rs.html#8-13) trait that is used in the bounds of the [`CustomizableOperation::send` call](https://docs.rs/aws-sdk-dynamodb/latest/src/aws_sdk_dynamodb/client/customize.rs.html#106-110) as public.

### Use Case

The reason that I would like to have this trait public, but not implementable is so that I can generically call [`CustomizableOperation::send` call](https://docs.rs/aws-sdk-dynamodb/latest/src/aws_sdk_dynamodb/client/customize.rs.html#106-110) without needing to know the concrete operation types.

For example:

```rust
async fn send_with_interceptor(
operation: CustomizableOperation,
) -> (Result>>, Option)
where
E: Error + Send + Sync + 'static,
B: aws_sdk_dynamodb::client::customize::internal::CustomizableSend,
// fails with "error[E0603]: module `internal` is private"
{
operation
.interceptor(SomeInterceptor)
.send()
.await
}
```

Overall, I would like to be able to do some things with the generic operations, like:
1. Apply an interceptor to the custom operation
2. Call `send()` and `.await` it

### Proposed Solution

I propose that this trait be made public. However, I wouldn't want this trait to be implemented by external crates, so it would be necessary to use a private `Seal` marker trait as a supertrait to prevent that.

### Acknowledgements

- [x] I may be able to implement this feature request (maybe)
- [ ] This feature might incur a breaking change

### A note for the community

### Community Note
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Start by reading the linked aws-sdk-dynamodb client/customize.rs source, especially CustomizableSend and CustomizableOperation::send, then inspect how the corresponding SDK traits expose these bounds. Done means generic operations can apply an interceptor and await send without concrete operation types, while external crates still cannot implement the public trait.

Written by the indexing model from the issue text.