feat(sdk,core): Use async runtime instead of blocking.

Default to using async runtime instead of blocking. I.e.

```rust
fn main() -> eyre::Result<()> {
  // ...

  client.container().from("rust").publish("somewhere")?;

  // ...
}

// to

async fn main() -> eyre::Result<()> {
  // ...

  client.container().from("rust").publish("somewhere").await?;

  // ...
}
```

crates/dagger-sdk/examples/first-pipeline/main.rs

@@ -1,11 +1,12 @@
-fn main() -> eyre::Result<()> {
+#[tokio::main]
+async fn main() -> eyre::Result<()> {
     let client = dagger_sdk::connect()?;
 
     let version = client
         .container()
         .from("golang:1.19")
-        .with_exec(vec!["go", "version".into()])
-        .stdout()?;
+        .with_exec(vec!["go", "version"])
+        .stdout().await?;
 
     println!("Hello from Dagger and {}", version.trim());