Multi-instance custom service cannot output services

This restriction is so because by design we can only output processes (via `outputs.settings`) not further services.

https://github.com/juspay/services-flake/blob/7200b458b5719eaf08c72eb7110b889f25b8e26f/nix/lib.nix#L43-L54

https://community.flake.parts/services-flake/custom-service#multi-instance

--

Whereas, from user standpoint, it makes sense to have a hierarchy of services. Say, a `infra-switch` service expands to both a postgres process and a `process-tracker` service, which then further expands to a Haskell process and its own postgres process. Here, we can define a custom service for `postgres-tracker` service, but not the `infra-switch` service, unless we give up on it being a multi-instance service.

```mermaid
graph TD
A[infra-switch service] --> B[postgres process]
A --> C[process-tracker service]
C --> D[Haskell process]
C --> E[postgres process]

style A fill:#f9f,stroke:#333,stroke-width:2px
style C fill:#ccf,stroke:#333,stroke-width:2px
style B fill:#cfc,stroke:#333,stroke-width:2px
style D fill:#cfc,stroke:#333,stroke-width:2px
style E fill:#cfc,stroke:#333,stroke-width:2px
```

In principle, we don't intend to run multiple instances of `infra-switch` ... however, making it multi-instance means we can leverage `name` in certain places, like `namespace` (cf. #258) and generally provide a consistent modular interface.