Property name namespaces

With v1.3 of the spec custom properties were introduced. But no real guidance on naming.

For example, CDX support is being added to Tern, a container SCA tool. One of the things Tern does is detect the OS, i.e. Debian. It refers to this information as `os_guess`. In that implementation this has been added as the property `tern:os_guess`.

This sort of namespacing can make it easier for folks trying to build tooling that processes CDX BOMs. They can now add support for `tern:os_guess`, if relevant to them, with less chance of a naming collision.

I also propose we have a standard way of namespacing CDX implementation specific properties. i.e. `cdx:dotnet:whatever`. Although that scheme should be defined at the CDX project level, the `whatever` can be delegated to each implementation.

Maybe there should be official package/build ecosystem namespaces too? i.e. `cdx:nuget:whatever` and `cdx:msbuild:whatever`? This could really help 3rd party tool implementers.

I think we should also specify a namespace for internal use only. We're probably all already throwing around CDX BOMs between our own internal tooling. But an `internal` namespace would mean that tooling, like the CLI, could support stripping properties that are only intended for internal usage before publishing the final "public" BOM.

So, I guess my proposal is:

- add documentation to the specification repo which details the high level recommended property namespacing scheme
- add references to that documentation in the description of properties in the spec itself
- create a new repo, cyclonedx-property-namespaces, with a file for each official CDX property namespace, i.e. `cdx:nuget` and a list of official NuGet specific property names (perhaps code owners can be used to delegate these to relevant implementation maintainers)