Refactor configuration schema in CAMARA_event_common.yaml

[rartych]

What type of PR is this?

• correction

What this PR does / why we need it:

Applied the Config → ConfigBase refactor across the Commonalities repo:

1. artifacts/common/CAMARA_event_common.yaml — Renamed Config to ConfigBase, removed the required subscriptionDetail property and its internal $ref, removed the now-unused CreateSubscriptionDetail stub.

2. artifacts/api-templates/sample-service-subscriptions.yaml — Added a locally-defined Config schema that extends ConfigBase via allOf and adds the required subscriptionDetail (typed against a newly added local CreateSubscriptionDetail stub). Updated the two config: property refs (in SubscriptionRequest and Subscription) to point to the local #/components/schemas/Config instead of the common file.

3. documentation/CAMARA-API-Event-Subscription-and-Notification-Guide.md — Added a note under the config attributes table explaining the ConfigBase/Config split and showing the allOf pattern API projects must follow, with a pointer to the updated template.

Which issue(s) this PR fixes:

Fixes #659

Does this PR introduce a breaking change?

• Yes
• No

Special notes for reviewers:

Changelog input

Refactored configuration schema in CAMARA_event_common.yaml 

Additional documentation

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docs