Move `BodyParameter` definition to Azure.Core templates

### Clear and concise description of the problem

To clearly describe spreading behavior, we are vendoring template `BodyParameter` in specs if we don't want spread body parameters. We want to eventually move this to `Azure.Core` so we don't need to vendor. The options are either adding the template itself, like

```tsp
model BodyParameter {
@friendlyName(name)
@bodyRoot body: T;
}
```

OR, by introducing a named parameter to templates, so spec authors would write this:
```tsp
op doSomething
```

### Checklist

- [X] Follow our [Code of Conduct](https://github.com/azure/typespec-azure/blob/main/CODE_OF_CONDUCT.md)
- [X] Check that this issue is about the Azure libraries for typespec. For feature request in the typespec language or core libraries file it in the [TypeSpec repo](https://github.com/Microsoft/TypeSpec/issues/new/choose)
- [X] Read the [docs](https://azure.github.io/typespec-azure/).
- [X] Check that there isn't already an issue that request the same feature to avoid creating a duplicate.