feat: add LinkedDatabase CRD

[tkelkermans]

Summary

• Add a namespaced LinkedDatabase CRD for creating logical PostgreSQL databases on an existing NDB database instance.
• Map reconciliation to POST /databases/{sourceDatabaseId}/linked-databases with body {"databases":[{"databaseName":"<name>"}]}.
• Support identifying the source instance by spec.sourceDatabaseId or by spec.sourceDatabaseName resolved from NDBServer status.
• Check existing linked databases before issuing create, so an already-present linked database is adopted into CR status instead of posted again.
• Add generated CRD/RBAC, Helm CRD/RBAC, API helpers, controller tests, and README documentation.

Scope notes

• This PR implements the create/observe flow only. Deleting a LinkedDatabase CR does not delete the linked database from NDB.

Verification

• make test
• bin/kustomize build config/default >/tmp/ndb-default.yaml
• helm lint deploy/helm

[manavrajvanshi]

Hi @tkelkermans

Is there any specific use-case that would require a separate linked-database CRD and controller specifically? Have you considered moving the linked-databases within the Databases custom resource instead? Were there any challenges in that approach?