Support custom retry classifier on generated clients

[DanielBauman88]

Rust does not support this (unless I've missed it).
https://docs.aws.amazon.com/sdk-for-rust/latest/dg/retries.html

RetryConfig only supports mode, max_attempts, and initial_backoff.

This is limiting and makes writing client code painful if custom retry strategies are needed. EG: If the desired client behaviour is retrying ddb conditonal check failed exceptions this can't be configured on the ddb client, the check has to be applied in code each time a call is made.

This limitation does not exist for the java sdk where arbitrary retry logic can be defined based on the request, response and number of retries.

1. It would be great to support custom retry behaviour like default + x in the rust generated clients.
2. Is there a hackier way to achieve this while the SDK doesn't support it in a first class way? I had some success doing something similar with the go sdk using custom middleware, but I haven't figured out how to apply custom middleware with the generated rust interfaces.

[DanielBauman88]

https://docs.rs/aws-smithy-client/latest/aws_smithy_client/struct.Builder.html#method.middleware_fn <- this might be the way.

[DanielBauman88]

It would be helpful to have a way to set the RetryClassifier that will be used by the client.
That would allow for a RetryClassifier to be set which makes use of the default and adds additional logic for retry.

Default: https://github.com/awslabs/smithy-rs/blob/main/rust-runtime/aws-smithy-http/src/retry.rs#L23