Make AppName configurable from within ConfigLoader (#2513)

* Add setter for `app_name` in `ConfigLoader`

This commit adds a setter for `app_name` in `ConfigLoader`. Prior to the
commit, the field was settable only from `SdkConfig` onward.

* Update CHANGELOG.next.toml

* Update CHANGELOG.next.toml

Co-authored-by: John DiSanti <jdisanti@amazon.com>

* Update CHANGELOG.next.toml

Co-authored-by: John DiSanti <jdisanti@amazon.com>

* Update aws/rust-runtime/aws-config/src/lib.rs

Co-authored-by: John DiSanti <jdisanti@amazon.com>

* Make empty string an invalid AppName

This commit addresses https://github.com/awslabs/smithy-rs/pull/2513#discussion_r1153074370



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Co-authored-by: Yuki Saito <awsaito@amazon.com>
Co-authored-by: John DiSanti <jdisanti@amazon.com>