Add the ability to write Dagger modules in Rust, matching the module
support available in Go, Python, TypeScript, and other SDKs.

SDK changes (sdk/rust/):
- crates/dagger-sdk: add `module` feature with registration, invocation,
  dag() global, gen_deps re-exports, and top-level prelude re-exports.
  Make querybuilder module public (needed by generated code).
- crates/dagger-sdk-derive: new proc macro crate providing
  #[dagger_module] and #[dagger_function] attribute macros
- runtime/: Go runtime module implementing ModuleRuntime() and Codegen()
  for the Dagger engine, with codegen via dagger-bootstrap
- dagger.json: root module config for engine SDK loading via GitHub
- runtime/dagger.json: standalone config for local path testing
- examples/test-module/: working example module

Engine changes (core/sdk/):
- Register "rust" as a built-in SDK using the external module pattern
  (same as Java/PHP/Elixir), resolving to github.com/dagger/dagger/sdk/rust

User code:
  use dagger_sdk::*;

  #[derive(Default)]
  pub struct MyModule;

  #[dagger_module]
  impl MyModule {
      #[dagger_function]
      fn hello(&self) -> String {
          "Hello from Rust!".to_string()
      }

      #[dagger_function]
      async fn build(&self, msg: String) -> eyre::Result<String> {
          dag().container().from("alpine:latest")
              .with_exec(vec!["echo", &msg])
              .stdout().await
              .map_err(|e| eyre::eyre!(e))
      }
  }

  #[tokio::main]
  async fn main() -> eyre::Result<()> {
      dagger_sdk::run(MyModule).await
  }

Signed-off-by: kjuulh <contact@kjuulh.io>