feat(sdk): re-export through lib.rs

this means that you can now use dagger_sdk::connect() instead of
dagger_sdk::client::connect();

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

@@ -1,5 +1,5 @@
 fn main() -> eyre::Result<()> {
-    let client = dagger_sdk::client::connect()?;
+    let client = dagger_sdk::connect()?;
 
     let version = client
         .container(None)