Improve docs no credentials (#3279)

## Motivation and Context
<!--- Why is this change required? What problem does it solve? -->
<!--- If it fixes an open issue, please link to the issue here -->
- aws-sdk-rust#971

## Description
Add documentation to `no_credentials` and add `test_credentials()`
method

## Testing
CI

## Checklist
- [x] I have updated `CHANGELOG.next.toml` if I made changes to the AWS
SDK, generated SDK code, or SDK runtime crates

----

_By submitting this pull request, I confirm that you can use, modify,
copy, and redistribute this contribution, under the terms of your
choice._

---------

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

Author: rcoh

CHANGELOG.next.toml

@@ -32,3 +32,20 @@ result of the compilation."""
 references = ["aws-sdk-rust#975", "smithy-rs#3269"]
 meta = { "breaking" = false, "tada" = true, "bug" = false }
 author = "jdisanti"
+
+[[aws-sdk-rust]]
+message = """Add `test_credentials` to `ConfigLoader` in `aws_config`. This allows the following pattern during tests:
+
+```rust
+async fn main() {
+    let conf = aws_config::defaults(BehaviorVersion::latest())
+        .test_credentials()
+        .await;
+}
+```
+
+This is designed for unit tests and using local mocks like DynamoDB Local and LocalStack with the SDK.
+"""
+meta = { "breaking" = false, "tada" = true, "bug" = false }
+author = "rcoh"
+references = ["smithy-rs#3279", "aws-sdk-rust#971"]

aws/rust-runtime/aws-config/Cargo.toml

@@ -20,7 +20,7 @@ credentials-process = ["tokio/process"]
 default = ["client-hyper", "rustls", "rt-tokio", "credentials-process", "sso"]
 
 [dependencies]
-aws-credential-types = { path = "../../sdk/build/aws-sdk/sdk/aws-credential-types" }
+aws-credential-types = { path = "../../sdk/build/aws-sdk/sdk/aws-credential-types", features = ["test-util"] }
 aws-http = { path = "../../sdk/build/aws-sdk/sdk/aws-http" }
 aws-sdk-sts = { path = "../../sdk/build/aws-sdk/sdk/sts", default-features = false }
 aws-smithy-async = { path = "../../sdk/build/aws-sdk/sdk/aws-smithy-async" }
@@ -52,7 +52,6 @@ zeroize = { version = "1", optional = true }
 aws-sdk-ssooidc = { path = "../../sdk/build/aws-sdk/sdk/ssooidc", default-features = false, optional = true }
 
 [dev-dependencies]
-aws-credential-types = { path = "../../sdk/build/aws-sdk/sdk/aws-credential-types", features = ["test-util"] }
 aws-smithy-runtime = { path = "../../sdk/build/aws-sdk/sdk/aws-smithy-runtime", features = ["client", "connector-hyper-0-14-x", "test-util"] }
 aws-smithy-runtime-api = { path = "../../sdk/build/aws-sdk/sdk/aws-smithy-runtime-api", features = ["test-util"] }
 futures-util = { version = "0.3.16", default-features = false }

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

@@ -212,6 +212,7 @@ mod loader {
     use crate::profile::profile_file::ProfileFiles;
     use crate::provider_config::ProviderConfig;
     use aws_credential_types::provider::{ProvideCredentials, SharedCredentialsProvider};
+    use aws_credential_types::Credentials;
     use aws_smithy_async::rt::sleep::{default_async_sleep, AsyncSleep, SharedAsyncSleep};
     use aws_smithy_async::time::{SharedTimeSource, TimeSource};
     use aws_smithy_runtime_api::client::behavior_version::BehaviorVersion;
@@ -458,6 +459,10 @@ mod loader {
         /// anonymous auth for S3, calling operations in STS that don't require a signature,
         /// or using token-based auth.
         ///
+        /// **Note**: For tests, e.g. with a service like DynamoDB Local, this is **not** what you
+        /// want. If credentials are disabled, requests cannot be signed. For these use cases, use
+        /// [`test_credentials`](Self::test_credentials).
+        ///
         /// # Examples
         ///
         /// Turn off credentials in order to call a service without signing:
@@ -474,6 +479,11 @@ mod loader {
             self
         }
 
+        /// Set test credentials for use when signing requests
+        pub fn test_credentials(self) -> Self {
+            self.credentials_provider(Credentials::for_tests())
+        }
+
         /// Override the name of the app used to build [`SdkConfig`](aws_types::SdkConfig).
         ///
         /// This _optional_ name is used to identify the application in the user agent that