Add a fully worked example for signing requests for API gateway

[rcoh]

Describe the issue

There are currently examples in the aws-sigv4 crate but they don't clearly articulate how to set up the end-to-end signing (e.g. obtaining credentials, producing a request, etc.

Links

https://docs.rs/aws-sigv4/latest/aws_sigv4/http_request/index.html#example-signing-an-http-request

[ymwjbxxq]

Also, remember to add an example without

let identity = Credentials::new(
    "AKIDEXAMPLE",
    "wJalrXUtnFEMI/K7MDENG+bPxRfiCYEXAMPLEKEY",
    None,
    None,
    "hardcoded-credentials"

maybe we link to #1037

[jmklix]

Closing as duplicate: #948

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