Add FetchCode support for cargo

[TG1999]

PURL: pkg:cargo/serde@1.0.136

Download URL:
https://crates.io/api/v1/crates/serde/1.0.136/download

[TG1999]

This is done in

https://github.com/package-url/packageurl-python/blob/main/src/packageurl/contrib/purl2url.py#L340

Commit:
package-url/packageurl-python@9ea1cea