Timeout settings are now merged instead of replaced

[rcoh]

Fix bug where setting TimeoutSettings would completely replace other timeout settings. Prior to this change, if a CONNECT timeout was set, for example, setting an operation_timeout would ignore this previously configured value.

This change will merge individual timeout settings. To disable a specific timeout, use TimeoutConfig::disabled or TimeoutConfigBuilder::disable_<xyz>.

This will cause a behavior change for customers who set custom timeout settings in combination with aws_config. aws-config provides a default connect timeout of 3.1s.

use std::time::Duration;

async fn main() {
    // BEFORE: no connect timeout
    // AFTER: `connect`  timeout of 3.1s.
    let conf = aws_config::defaults(BehaviorVersion::latest())
            .timeout_config(TimeoutConfig::builder().operation_timeout(Duration::from_secs(3)).build())
            .load().await;
}

A 3.1 second connect timeout is generally a best practice for non-mobile environments so this change should not be noticeable by most customers.