Apache CycloneDX Ant Library - externalreferenceset

## externalreferenceset

externalreferencesets are collections of external references (typed links) that can be associated to a component or an SBOM. They can be used as top-level elements and be given an id so they can be later referred to via the `refid` attribute - see the Ant manual.

Apart from Ant's `id/refid` this element doesn't support any attributes.

### Nested elements

#### externalReference

Represents a single external reference to be added to the BOM.

Attribute | Description | Required
--- | --- | ---
type | The type of external references. The valid values are defined by the CycloneDX specification. | Yes
url | The url of the external reference. | Yes

### Examples

Below is a set of external references this Ant Library could use for itself.

          <cdx:externalreferenceset
            id="antlib-ext-refs"
            xmlns:cdx="antlib:org.apache.ant.cyclonedx">
            <externalReference
                type="vcs"
                url="https://github.com/apache/ant-antlibs-cyclonedx"/>
            <externalReference
                type="license"
                url="https://www.apache.org/licenses/LICENSE-2.0.txt"/>
            <externalReference
                type="build-system"
                url="https://ci-builds.apache.org/job/Ant/job/CycloneDX%20Antlib/"/>
            <externalReference
                type="mailing-list"
                url="https://ant.apache.org/mail.html"/>
            <externalReference
                type="issue-tracker"
                url="https://bz.apache.org/bugzilla/buglist.cgi?component=CycloneDX%20Antlib&product=Ant"/>
            <externalReference
                type="website"
                url="https://ant.apache.org/antlibs/cyclonedx/"/>
            <externalReference
                type="distribution"
                url="https://ant.apache.org/antlibs/bindownload.cgi"/>
            <externalReference
                type="source-distribution"
                url="https://ant.apache.org/antlibs/srcdownload.cgi"/>
            <externalReference
                type="security-contact"
                url="https://www.apache.org/security/"/>
          </cdx:externalreferenceset>