Discourage use of hardcoded credentials (#875)

**TODO Upon release**

- Update README & aws-sdk-rust CI for MSRV upgrade to 1.54

**New this week**

**New this release**

- :tada: Timeouts for requests are now configurable. You can set a timeout for each individual request attempt or for all attempts made for a request. (smithy-rs#831)

- Improve docs on `aws-smithy-client` (smithy-rs#855)

**Breaking changes**

- `RetryConfigBuilder::merge_with` has been renamed to `RetryConfigBuilder::take_unset_from`

- `Credentials::from_keys` is now behind a feature flag named `hardcoded-credentials` in `aws-types`.

  It is __NOT__ secure to hardcode credentials into your application, and the credentials

  providers that come with the AWS SDK should be preferred. (smithy-rs#875, smithy-rs#317)

- (aws-smithy-client): Extraneous `pub use SdkSuccess` removed from `aws_smithy_client::hyper_ext`. (smithy-rs#855)

v0.0.26-alpha (TBD)

===================================

**New this release**

- Improve docs on `aws-smithy-client` (smithy-rs#855)

**Breaking Changes**

- (aws-smithy-client): Extraneous `pub use SdkSuccess` removed from `aws_smithy_client::hyper_ext`. (smithy-rs#855)

**Breaking Changes**

        }

        /// Override the credentials provider used to build [`Config`](aws_types::config::Config).

        ///

        /// # Examples

        ///

        /// Override the credentials provider but load the default value for region:

        /// ```rust

        /// # use aws_types::Credentials;

        ///  async fn create_config() {

        /// # fn create_my_credential_provider() -> Credentials {

        /// #     Credentials::new("example", "example", None, None, "example")

        /// # }

        /// # async fn create_config() {

        /// let config = aws_config::from_env()

        ///     .credentials_provider(Credentials::from_keys("accesskey", "secretkey", None))

        ///     .load().await;

        ///     .credentials_provider(create_my_credential_provider())

        ///     .load()

        ///     .await;

        /// # }

        /// ```

        pub fn credentials_provider(

/// * Finally, if a provider returns any other error condition, an error will be returned immediately.

///

/// # Examples

///

/// ```rust

/// use aws_config::meta::credentials::CredentialsProviderChain;

/// use aws_types::Credentials;

/// use aws_config::environment;

/// use aws_config::environment::credentials::EnvironmentVariableCredentialsProvider;

/// use aws_config::profile::ProfileFileCredentialsProvider;

///

/// let provider = CredentialsProviderChain::first_try("Environment", EnvironmentVariableCredentialsProvider::new())

///     .or_else("Static", Credentials::from_keys("someacceskeyid", "somesecret", None));

///     .or_else("Profile", ProfileFileCredentialsProvider::builder().build());

/// ```

#[derive(Debug)]

pub struct CredentialsProviderChain {

    async fn provide_credentials_fn_closure_can_borrow() {

        fn check_is_str_ref(_input: &str) {}

        async fn test_async_provider(input: String) -> credentials::Result {

            Ok(Credentials::from_keys(&input, &input, None))

            Ok(Credentials::new(&input, &input, None, None, "test"))

        }

        let things_to_borrow = vec!["one".to_string(), "two".to_string()];

    /// let provider = LazyCachingCredentialsProvider::builder()

    ///     .load(provide_credentials_fn(|| async {

    ///         // An async process to retrieve credentials would go here:

    ///         Ok(Credentials::from_keys("example", "example", None))

    ///         Ok(Credentials::new("example", "example", None, None, "my_provider_name"))

    ///     }))

    ///     .build();

    /// ```