feat: allow scoped spreadsheet imports

[marianfoo]

Summary

Fixes #1 by adding an opt-in scope for spreadsheet imports.

• Adds support for @spreadsheetimporter.enabled on CDS entities and service projections.
• Keeps existing behavior when no entities are annotated, so current projects continue to allow all imports by default.
• When at least one entity is annotated, rejects non-enabled entities with 403 SPREADSHEET_IMPORT_ENTITY_NOT_ENABLED before reading/parsing the upload stream.
• Resolves annotated service projections to their persistence entity so existing URLs such as Spreadsheet(entity='my.bookshop.Books')/content keep working.
• Preserves CAP entity lookup behavior for short names such as Books via cds.entities() while using compiled model definitions for projection/source resolution.
• Documents that the plugin annotation controls target-entity eligibility only; user/role authorization should stay in CAP annotations such as @requires or @restrict.
• Adds unit coverage plus CAP OData integration coverage for fully qualified names, service projection names, short names, disabled entities, and unknown entities.

Root Cause

The importer previously trusted the entity URL parameter, resolved it directly from the CAP entity lookup, and inserted through cds.db. That bypassed normal service-level restrictions such as @readonly on exposed service entities, so there was no way to enable the importer for only selected entities.

CAP Practice Applied

• Model-level eligibility is expressed through a CDS annotation, matching CAP's annotation-driven modeling style.
• Role/user authorization is intentionally left to CAP's built-in @requires and @restrict annotations instead of being reimplemented in the importer.
• Request failures use req.reject(...), which CAP documents as the preferred request rejection path.
• Tests use cds.test(...).in(...) so CAP test servers are isolated and no external localhost server is required.

Validation

• node -c importer-service.js
• git diff --check
• npm test -- --runInBand — 3 suites, 12 tests passing