add wstd-aws crate, for integration with aws-sdk

[pchickey]

wstd-aws crate

This crate provides support for using the AWS Rust SDK for the wasm32-wasip2
target using the wstd crate.

In many wasi settings, its necessary or desirable to use the wasi-http
interface to make http requests. Wasi-http interfaces provide an http
implementation, including the sockets layer and TLS, outside of the user's
component. wstd provides user-friendly async Rust interfaces to all of the
standardized wasi interfaces, including wasi-http.

The AWS Rust SDK, by default, depends on tokio, hyper, and either rustls
or s2n_tls, and makes http requests over sockets (which can be provided as
wasi-sockets). Those dependencies may not work correctly under wasm32-wasip2,
and if they do, they will not use the wasi-http interfaces. To avoid using
http over sockets, make sure to set the default-features = false setting
when depending on any aws-* crates in your project.

To configure wstd's wasi-http client for the AWS Rust SDK, provide
wstd_aws::sleep_impl() and wstd_aws::http_client() to your
aws_config::ConfigLoader:

    let config = aws_config::defaults(BehaviorVersion::latest())
        .sleep_impl(wstd_aws::sleep_impl())
        .http_client(wstd_aws::http_client())
        ...;

Example

An example s3 client is provided as a wasi cli command. It accepts command
line arguments with the subcommand list to list a bucket's contents, and
get <key> to get an object from a bucket and write it to the filesystem.

This example must be compiled in release mode - in debug mode, the aws
sdk's generated code will overflow the maximum permitted wasm locals in
a single function.

Compile it with:

cargo build -p wstd-aws --target wasm32-wasip2 --release --examples

When running this example, you will need AWS credentials provided in environment
variables.

Run it with:

wasmtime run -Shttp \
    --env AWS_ACCESS_KEY_ID \
    --env AWS_SECRET_ACCESS_KEY \
    --env AWS_SESSION_TOKEN \
    --dir .::. \
    target/wasm32-wasip2/release/examples/s3.wasm

or alternatively run it with:

cargo run --target wasm32-wasip2 -p wstd-aws --example s3

which uses the wasmtime cli, as above, via configuration found in this
workspace's .cargo/config.

By default, this script accesses the wstd-example-bucket in us-west-2.
To change the bucket or region, use the --bucket and --region cli
flags before the subcommand.

CI

The example is tested in CI, which means CI needs aws credentials. I have configured a role and bucket on my AWS account to use a OIDC-based authentication which trusts GitHub Actions run on the bytecodealliance/wstd repo. If the CI workflow is run locally, or on other repos where this authentication fails, the test-programs crate has feature no-aws that will #[ignore] the tests which require aws credentials.

[yoshuawuyts]

Looked this over; looks great! Tysm!