This PR implements a simple HTTP server to serve static files, so tests no longer depend on remote files.

Example:

#[test]
fn test_semconv_spec_from_url() {
    let server = ServeStaticFiles::from("tests/test_data").unwrap();
    let semconv_url = server.relative_path_to_url("url/common.yaml");
    let result =
        SemConvRegistry::semconv_spec_from_url(&semconv_url).into_result_failing_non_fatal();
    assert!(result.is_ok());
}

In this example, the mini HTTP server returns the file located at tests/test_data/url/common.yaml. The URL returned by server.relative_path_to_url is something like http://127.0.0.1:<dyn_port>/url/common.yaml.

Closes: #267