make ref/bom-ref own data type in schema

[jkowalleck]

motivation:

in current(spec<=1.3, 1.4-dev) schema files the definition of ref and bom-ref are defined as an arbitrary string.
Basically both are simple strings, but a ref should be a value that was used as a bom-ref in another place, so both strings are from the same "string-class" and unlike other arbitrary strings.
The fact that both strings are interlinked is made clear in comments, but not in the DataType itself.

To make the relation between these values more clear, i would love to introduce an schema-based string type that extends "string" and is used for bom-ref-based object-interlinks.

scope

schema 1.1 XML
schema 1.2 XML & JSON
schema 1.3 XML & JSON
schema 1.4-dev XML & JSON

acc / crit:

• changes to the schema may add additional documentations
• changes to the schema must not change any structure or data-types.
• changes to the schema must not add any new features on data-level

implementation details

• call new DataType "refType"
• new DataType must inherit "string"-type
• new DataType should be made publicly available, so other schemas or extensions can use it to point out that elements are interlinked.

[stevespringett]

Is this still an issue? Changes to prior schemas will not be made, but if this issue is still present in v1.5-dev branch, we have time to correct it (assuming doing so is backward compatable).

[jkowalleck]

@stevespringett re: #114 (comment)

✔️ The feature was already introduced in v1.4 via #116.

Additionally, #115 would back-port the features into older specs without breaking changes. Take it or leave it, you decide.
Feel free to close this PR and mark #115 as "closed as not planned".

PS: The refType is just an alias of string with an annotation.
Therefore, I propose to merge this without any version bump.

[jkowalleck]

will close it.
for me (the author of this ticket) the currently implemented fixes are good enough