aws-smithy-wasm: add impl Default for WasiHttpClient (#4563)

## Motivation and Context

Prior to this PR, the only way to construct a `WasiHttpClient` is via
`WasiHttpClientBuilder::new().build()`. There are no other options on
the Builder. The Builder exists to provide a surface for future
expansion, and because #4498 was designed to minimize diff when changing
the implementation behind the WasiHttpClient type.

## Description

Adds `impl Default for WasiHttpClient` containing the trivial
construction.

## Testing
The existing test in `tools/ci-cdk/canary-wasm` has been modified to
exercise this new constructor.


## Checklist
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rather than leaving it unchecked -->
- [ ] For changes to the smithy-rs codegen or runtime crates, I have
created a changelog entry Markdown file in the `.changelog` directory,
specifying "client," "server," or both in the `applies_to` key.
- [ ] For changes to the AWS SDK, generated SDK code, or SDK runtime
crates, I have created a changelog entry Markdown file in the
`.changelog` directory, specifying "aws-sdk-rust" in the `applies_to`
key.

----

_By submitting this pull request, I confirm that you can use, modify,
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choice._

---------

Co-authored-by: ysaito1001 <awsaito@amazon.com>

Author: pchickey

rust-runtime/aws-smithy-wasm/Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "aws-smithy-wasm"
-version = "0.1.10"
+version = "0.1.11"
 authors = [
   "AWS Rust SDK Team <aws-sdk-rust@amazon.com>",
   "Eduardo Rodrigues <16357187+eduardomourar@users.noreply.github.com>",

rust-runtime/aws-smithy-wasm/src/wasi.rs

@@ -57,7 +57,7 @@ impl WasiHttpClientBuilder {
 /// An HTTP client that can be used during instantiation of the client SDK in
 /// order to route the HTTP requests through the WebAssembly host. The host must
 /// support the wasi-http interface as defined in the WASIp2 specification.
-#[derive(Debug, Clone)]
+#[derive(Debug, Default, Clone)]
 #[non_exhaustive]
 pub struct WasiHttpClient {}
 

tools/ci-cdk/canary-wasm/src/lib.rs

@@ -5,7 +5,7 @@
 
 use aws_config::Region;
 use aws_sdk_s3 as s3;
-use aws_smithy_wasm::wasi::{WasiHttpClientBuilder, WasiSleep};
+use aws_smithy_wasm::wasi::{WasiHttpClient, WasiSleep};
 
 //Generates the Rust bindings from the wit file
 wit_bindgen::generate!({
@@ -23,7 +23,7 @@ impl exports::aws::component::canary_interface::Guest for Component {
 }
 
 async fn run() -> Result<Vec<String>, String> {
-    let http_client = WasiHttpClientBuilder::new().build();
+    let http_client = WasiHttpClient::default();
     let sleep = WasiSleep;
     let config = aws_config::from_env()
         .region(Region::new("us-east-2"))