Override [provider] functions to let users use it without context

```
#[provider(singleton = true)]
fn provide_text_assembler(_: &mut Context<ApplicationConfig>) -> TextAssemblerRef {
    Arc::new(TextAssembler::new())
}

#[provider(singleton = true)]
```

would let users do

```
#[provider(singleton = true)]
fn provide_text_assembler() -> TextAssemblerRef {
    Arc::new(TextAssembler::new())
}

#[provider(singleton = true)]
```