Align with CycloneDX taxonomy

I would support this PR. only IFF the values of specific cert. types are defined as part an official cdx property tax. (i.e., one reserved here: https://github.com/CycloneDX/cyclonedx-property-taxonomy/blob/main/cdx.md). These values are essential for use within any externalReference entry for providing the name of the property and the value (in place of an emum. in the base CDX spec.).

_Originally posted by @mrutkows in https://github.com/CycloneDX/transparency-exchange-api/pull/216#discussion_r2853864733_