All forms of SigningParams now take an Identity instead of Credentials

[Velfi]

All versions of SigningParams have been updated to contain an Identity
as opposed to AWS credentials in &str form. If your code broke as a result of this change, the fix is easy: Instead of passing credentials directly
to a signing function, wrap them in an Identity first:

use aws_runtime::auth::sigv4::SigV4Signer;
use aws_credential_types::Credentials;
use aws_smithy_runtime_api::client::identity::Identity;
use std::time::{Duration, SystemTime};

let expiration = SystemTime::now() + Duration::from_secs(3600);
let your_creds = Credentials::new(
    "your_access_key_id",
    "your_secret_access_key",
    None,
    expiration.into(),
    "hardcoded credentials",
)
let identity = Identity::new(
    your_creds,
    expiration
);

// Creating the required config structs to sign with...

let signer = SigV4Signer::new();
let signing_params = signer.sign_http_request(
    settings,
    identity, // <- this used to be `credentials: &Credentials`
    &operation_config,
    request_timestamp,
)?;