Clarify that non-aws-config based config Builders don't have good defaults

[Mark-Simulacrum]

Describe the issue

Customers using e.g. aws_sdk_sts::config::Builder::new() and customizing a few settings (e.g., credential provider) don't currently receive defaults for other fields, such as the retry policy.

Ideally we'd have strong warnings against usage of these methods in favor of the aws-config constructors, and potentially consider some stronger messaging (e.g., deprecating in favor of without_defaults() or similarly less "easy to use" methods). Note that there's a bunch of ways to get such a builder today:

• https://docs.rs/aws-sdk-sts/latest/aws_sdk_sts/config/struct.Builder.html#method.new
• https://docs.rs/aws-sdk-sts/latest/aws_sdk_sts/config/struct.Config.html#method.builder

Each should likely have similar treatment. The docs on the Config struct also say:

The service config can also be constructed manually using its builder.

Which probably should be clarified to say "and this is almost certainly not what you want".

Links

(Also present in every other SDK):

• https://docs.rs/aws-sdk-sts/latest/aws_sdk_sts/config/struct.Builder.html#method.new
• https://docs.rs/aws-sdk-sts/latest/aws_sdk_sts/config/struct.Config.html#method.builder

[landonxjames]

Added a warning to those methods in smithy-lang/smithy-rs#3960, but we will also discuss marking them as deprecated going forward. Leaving this issue open pending those discussions.

[Mark-Simulacrum]

https://docs.rs/aws-config/latest/aws_config/imds/client/struct.Builder.html might also be an interesting case of a special client that seems like it can't be created from the aws-config Config? (That might make sense, since there's recursive dependency implied there for at least some of the configuration).

It might be a good idea to update its documentation if it's Builder is special and does come with the recommended defaults -- that looks true to me from a quick glance at the code, but don't trust me on that :)