CX-0012: Add Script Code Metadata Entity

[nicoprow]

Summary

• Adds a new metadata entity Script Code to §1.5.2 of CX-0012, defining it as a read-only reference list of writing systems available for use by implementations (e.g. Hans for Simplified Chinese).
• The actual codes are operator-defined; the standard does not prescribe a fixed set or mandate ISO 15924 compliance.
• Adds GET /script-codes as a required resource to the Metadata Controller (§2.2.1.4).

Context

Script codes support the script variant feature introduced in CX-0074 (Business Partner Data Gate API), where legal entities, sites, and addresses can carry name and address fields in multiple writing systems simultaneously. The Pool exposes the available script codes so that Gate consumers can discover which writing systems the operator supports.

Change Request

https://github.com/catenax-eV/product-standardization-prod/issues/666