Custom registries cause warning: Unknown source kind sparse

[kornelski]

WARN cargo_cyclonedx::purl] Unknown source kind sparse

cyclonedx-rust-cargo/cargo-cyclonedx/src/purl.rs

Line 17 in 22cac58

if let Some(source) = &package.source {

Packages from custom registries using Cargo's HTTP sparse protocol will have a source like: sparse+https://example.com/index/.

The same applies to package IDs. They will look like sparse+https://example.com/index/#crate@ver.