How to deal with externalReferences which aren't URI's (e.g. git@ refs)

## Problem

The `externalReference` in model is limited to URI's but according to the [Maven POM schema](https://maven.apache.org/xsd/maven-4.0.0.xsd) the SCM `url` is a simple string!
As a user of the `org.cyclonedx.bom` gradle plugin we loose the VCS information which is included in the component POM file.

Popular examples are the following components:
- https://repo1.maven.org/maven2/com/zaxxer/HikariCP/4.0.2/HikariCP-4.0.2.pom
- https://repo1.maven.org/maven2/com/zaxxer/SparseBitSet/1.2/SparseBitSet-1.2.pom

Could the schema be adapted or does it make sense to adapt the plugin to skip the BOM serialization according to the schema?
The information is necessary to retrieve the sources for further processing.

Thanks a lot for your efforts!