Add schema file name accessor to each DOM type C++ class

## Desired behavior

Getting the schema file for each SDF DOM type element should be possible.
This should make initializing an element for loading standalone DOM types easier.

Without this format. I am doing something like this in my code to initialize a DOM type element so that I can get type checking
```
// Schema file type
template
struct SdfTypeToSchemaFile {
static const std::string kSchemaFile;
};

template <>
const std::string SdfTypeToSchemaFile::kSchemaFile = "surface.sdf";
// Other definitions with template specialization.

template
sdf::ElementPtr init_element_for_type(){
auto element = std::make_shared<::sdf::Element>();
sdf::initFile(SdfTypeToSchemaFile::kSchemaFile, element);
return element;
}
```

Ideally, I hope to do something like this for simplicity.
```
template
sdf::ElementPtr init_element_for_type(){
auto element = std::make_shared<::sdf::Element>();
sdf::initFile(T::SchemaFile(), element);
return element;
}
```

## Alternatives considered

## Implementation suggestion

Implement a static function for each DOM type. For example for `Surface`:
```
// sdformat/include/sdf/Surface.hh
class Surface{
// Omitted details
public: static const std::string& SchemaFile();
};

// sdformat/src/Surface.cc

static const std::string& Surface::SchemaFile(){
static const std::string* kSchemaFile = new std::string("surface.sdf");
return *kSchemaFile;
}

```

## Additional context

Currently the schema file is hard coded in it's `T::ToElement()` for example:
https://github.com/gazebosim/sdformat/blob/c4bfe35bdaca8e667089bf9adf99b7930192ccaf/src/Surface.cc#L412