Introduce type for BOM-Link

[jkowalleck]

caused by #217

Introduction of bomLink types,
so that they can be used where needed, later.

This should allow formulating the answer for #136 and #217 in the schema,
so that there would be clarity.

[stevespringett]

This looks good to me. First thing I verified was that anchors were correctly used in the JSON schema and correctly omitted from the XSD. So technically, I think the PR is sound.

[jkowalleck]

I am uncertain about the ProtoBuff. Unclear whether to introduce the types there, too. (I did not find how union types are defined in ProtoBuff)
What do you think, @stevespringett, should we add there, too ?

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i'd suggest to merge this then,
and after the merge I'll prepare followup-PRs to formulate/clarify which elements allow BOM-Link as a formal value; I will prepare initial branch and PR for this, but might need assistance to finalize it.

[stevespringett]

I don't think regex is supported in protobuf. I belive we are omitting serialNumber, CPE, and a few other validations because of this.

Ok to merge.