Add --retries flag for API queries and downloads

ponyup is heavily used in automated CI across ponylang repos. Transient Cloudsmith failures (API query timeouts, download failures) shouldn't fail an entire build.

Add a `--retries` flag to `ponyup update` that retries both the Cloudsmith API query and the package download on failure. Default to 0 (no retries, current behavior). Each retry should include a short delay between attempts.