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/existing-dockerfile/main.rs

@@ -1,6 +1,7 @@
 use rand::Rng;
 
-fn main() -> eyre::Result<()> {
+#[tokio::main]
+async fn main() -> eyre::Result<()> {
     let mut rng = rand::thread_rng();
 
     let client = dagger_sdk::connect()?;
@@ -11,8 +12,8 @@ fn main() -> eyre::Result<()> {
 
     let ref_ = client
         .container()
-        .build(context_dir.id()?)
-        .publish(format!("ttl.sh/hello-dagger-rs-{}:1h", rng.gen::<u64>()))?;
+        .build(context_dir.id().await?)
+        .publish(format!("ttl.sh/hello-dagger-rs-{}:1h", rng.gen::<u64>())).await?;
 
     println!("published image to: {}", ref_);