multiService: Add `depends_on` as a default option

`multiService` defines a few [options defaults](https://github.com/juspay/services-flake/blob/4104b44868206c7fd7738e349d814a352c480711/nix/lib.nix#L18-L30) like `enable`, `namespace` and `dataDir`, which can reasonably expected to be defined by all the services.

I propose to also include `depends_on` here.

## Why?

- Often we end up splitting `depends_on` config for a service away from the service config, see

```nix
{
open-webui."open-webui1" = {
enable = true;
…
};

# Start the Open WebUI service after the Ollama service has finished initializing and loading the models
settings.processes.open-webui1.depends_on.ollama1-models.condition = "process_completed_successfully";
}
```
[(code snippet from example/llm)](https://github.com/juspay/services-flake/blob/4104b44868206c7fd7738e349d814a352c480711/example/llm/flake.nix#L49-L75)

If this proposal is implemented, it can be changed to:
```nix
{
open-webui."open-webui1" = {
enable = true;
depends_on.ollama1-models.condition = "process_completed_successfully";
…
};
}
```

- `depends_on` is an option almost always used while defining complex service dependency relationship.