Use local schema for add-on JAXB generation

[wborn]

Description

The build log of #5720 shows that JAXB/XJC timed out while downloading:

https://openhab.org/schemas/config-description-1.0.0.xsd

The failed import subsequently caused errors such as:

undefined simple type 'config-description:idRestrictionPattern'

This PR adds an XML catalog to org.openhab.core.addon so that the schema is resolved from the local org.openhab.core.config.core module instead.

This follows the same approach already used by the org.openhab.core.thing bundle.

Benefits

• Prevents JAXB generation failures when openhab.org is temporarily unavailable.
• Removes an unnecessary network dependency from the build.
• Reduces requests to openhab.org during CI and local builds.
• Keeps the canonical public schema URL in the XSD.
• Does not change the generated Java sources.

Testing

The change can be tested easily by building the bundle without network access, or by specifically blocking access to openhab.org, and verifying that JAXB source generation still succeeds.

• Compared src/generated/java before and after the change; no differences were found.
• Confirmed that JAXB source generation succeeds while access to openhab.org is blocked.

See also: #5731