Support referencing signal groups from dependent registries

Supporting multiple registries introduces new requirements and some additional complexity. This issue proposes addressing a specific need related to groups.

When users define a custom registry, they can currently add the standard OTEL registry as a dependency. This allows them to define new metrics, events, or spans by combining locally defined attributes with attributes already defined in the OTEL registry (via attribute refs).

It would also be beneficial to reference entire groups (e.g. an existing OTEL metric group) directly from the custom registry, allowing users to explicitly associate existing OTEL metrics with their own applications, services, or devices. Currently, the semantic convention format doesn’t support this, groups can only be defined with a unique id.

This issue proposes extending the existing attribute referencing mechanism to support referencing entire signal groups from dependent registries. Additionally, as a possible future enhancement, we might support overriding parts of these referenced groups, similar to how attribute references can currently be overridden.