Add FetchCode support for gem

[TG1999]

PURL: pkg:gem/rails@6.1.3.2

Download URL:
https://rubygems.org/downloads/rails-6.1.3.2.gem

[TG1999]

This is done in

https://github.com/package-url/packageurl-python/blob/main/src/packageurl/contrib/purl2url.py#L355

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