render property `cdx:reproducible`

[jkowalleck]

Is your feature request related to a problem? Please describe.

If a BOM was generated as reproducible, this should be easily visible from the BOM.
Therefore, CycloneDX/cyclonedx-property-taxonomy#70 exists

Describe the solution you'd like

Property cdx:reproducible is added under metadata.properties.
Value is true, if BOM was build in reproducible mode, else value is false.

Describe alternatives you've considered

Property cdx:reproducible could also be added under global properties, which exists since CDX1.5.
This global space does not exist as long as metadata.properties, which exists since CDX1.3.
For a better compatibility version-downwards, let's use the area that exists longer.

Additional context

Idea: use the metadata.properties, because the metadata also houses the timestamp of document creation.