Retry/timeout client API ergonomics and consistency

[jdisanti]

Some notes from playing around with manually configuring timeouts and retries with aws-config:

• aws_smithy_client::Builder's set_* vs non-set_* functions are inconsistent. The sleep_impl doesn't have a set_ prefix, but the retry and timeout configs do (and don't have a non-set_ prefix).
• Retry has aws_config::RetryConfig, but timeouts have aws::timeout::Config
• Timeout config should be re-exported along side with RetryConfig in the generated crate
  • This re-export should happen for both Smithy and AWS clients
• RetryConfig should be re-exported for both Smithy and AWS clients (currently only exported for AWS clients)
• Configuring timeouts requires importing TriState from aws_smithy_types. It should be re-exported if it really is required for timeout configuration
• The TriState for configuring timeouts is a little obtuse. We should follow the consistent pattern of thing/set_thing that the rest of the SDK uses where thing gives thing a value, and set_thing allows for removing that value (or directly setting the TriState in this case).

[jdisanti]

All these issues were resolved in #1740.