Add on-demand package data collection for cargo

[chinyeungli]

We want to enable the collect/ endpoint for cargo to avoid failure to collect a cargo PURL like pkg:cargo/purl :

{
  "status": "cannot fetch Package data for pkg:cargo/purl: no available handler"
}

Samples:
https://crates.io/crates/purl
API Link: https://crates.io/api/v1/crates/purl

https://github.com/aboutcode-org/scancode-toolkit/blob/develop/src/packagedcode/cargo.py maybe useful?