Closes #21356: Implement ETag support for REST API

[jeremystretch]

Closes: #21356

netbox/netbox/api/viewsets/__init__.py

1. ETagMixin class (new, lines 37–56): Static helper _get_etag() returns a quoted ISO 8601 ETag derived from last_updated (falling back to created). The retrieve() override appends the ETag header to detail responses.
2. NetBoxReadOnlyModelViewSet and NetBoxModelViewSet: ETagMixin added as the first base class, giving its retrieve() priority in the MRO over DRF's RetrieveModelMixin.retrieve().
3. update(): Added an If-Match precondition check per RFC 9110 §13.1.1 — if the header is present and not *, it compares against the current ETag and returns 412 Precondition Failed on mismatch. The response now includes a
   fresh ETag reflecting the updated last_updated value.
4. create(): Single-object creation responses now include an ETag header. Bulk creation responses do not (no single canonical object).

netbox/utilities/testing/api.py

1. test_get_object(): After the 200 OK assertion, verifies that ChangeLoggingMixin models return an ETag header.
2. test_update_object_with_etag() (new method): End-to-end test that verifies correct-ETag PATCH returns 200 with a changed ETag, and stale-ETag PATCH returns 412.