Add Apache 2.0 license

- Add LICENSE file with Apache 2.0 text
- Add license field to Cargo.toml
- Add copyright and SPDX headers to all source files
- Add License section to README.md

README.md

@@ -68,3 +68,7 @@ async fn run() -> Result<(), Box<dyn std::error::Error>> {
     Ok(())
 }
 ```
+
+## License
+
+This project is licensed under the [Apache License 2.0](LICENSE).