Add RDS IAM token example to aws-sig-auth (#811)

**New this week**

- Fix `native-tls` feature in `aws-config` (aws-sdk-rust#265, smithy-rs#803)

- :bug: Fix `native-tls` feature in `aws-config` (aws-sdk-rust#265, smithy-rs#803)

- Add example to aws-sig-auth for generating an IAM Token for RDS (smithy-rs#811, aws-sdk-rust#147)

v0.0.22-alpha (October 20th, 2021)

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

//! AWS Signature Authentication Package

//!

//! In the future, additional signature algorithms can be enabled as Cargo Features.

//! This crate may be used to generate presigned URLs for unmodeled behavior such as `rds-iam-token`.

//!

//! # Examples

//!

//! Generate RDS IAM Token:

//! ```rust

//! use aws_smithy_http::body::SdkBody;

//! use aws_types::region::{Region, SigningRegion};

//! use aws_types::{Credentials, SigningService};

//! use std::time::{Duration, SystemTime, UNIX_EPOCH};

//! use aws_sig_auth::signer::{self, SigningError, OperationSigningConfig, HttpSignatureType, RequestConfig};

//! fn generate_rds_iam_token(

//!     db_hostname: &str,

//!     region: Region,

//!     port: u16,

//!     db_username: &str,

//!     credentials: &Credentials,

//!     timestamp: SystemTime,

//! ) -> Result<String, SigningError> {

//!     let signer = signer::SigV4Signer::new();

//!     let mut operation_config = OperationSigningConfig::default_config();

//!     operation_config.signature_type = HttpSignatureType::HttpRequestQueryParams;

//!     operation_config.expires_in = Some(Duration::from_secs(15 * 60));

//!     let request_config = RequestConfig {

//!         request_ts: timestamp,

//!         region: &SigningRegion::from(region),

//!         service: &SigningService::from_static("rds-db"),

//!         payload_override: None,

//!     };

//!     let mut request = http::Request::builder()

//!         .uri(format!(

//!             "http://{db_hostname}:{port}/?Action=connect&DBUser={db_user}",

//!             db_hostname = db_hostname,

//!             port = port,

//!             db_user = db_username

//!         ))

//!         .body(SdkBody::empty())

//!         .expect("valid request");

//!     let _signature = signer.sign(

//!         &operation_config,

//!         &request_config,

//!         &credentials,

//!         &mut request,

//!     )?;

//!     let mut uri = request.uri().to_string();

//!     assert!(uri.starts_with("http://"));

//!     let uri = uri.split_off("http://".len());

//!     Ok(uri)

//! }

//!

//! let token = generate_rds_iam_token(

//!     "prod-instance.us-east-1.rds.amazonaws.com",

//!     Region::from_static("us-east-1"),

//!     3306,

//!     "dbuser",

//!     &Credentials::from_keys("AKIDEXAMPLE", "secret", None),

//!     // this value is hard coded to create deterministic signature for tests. Generally,

//!     // `SystemTime::now()` should be used

//!     UNIX_EPOCH + Duration::from_secs(1635257380)

//! ).expect("failed to generate token");

//! # // validate against token generated by the aws CLI

//! # assert_eq!(token, "prod-instance.us-east-1.rds.amazonaws.com:3306/?Action=connect&DBUser=dbuser&X-Amz-Algorithm=AWS4-HMAC-SHA256&X-Amz-Credential=AKIDEXAMPLE%2F20211026%2Fus-east-1%2Frds-db%2Faws4_request&X-Amz-Date=20211026T140940Z&X-Amz-Expires=900&X-Amz-SignedHeaders=host&X-Amz-Signature=9632f5f4fcd2087a3c523f55f72d2fe97fad03b71a0a23b8c1edfb104e8072d1");

//! ```

#[cfg(feature = "sign-eventstream")]

pub mod event_stream;