Blazor RenderFragment attributes

### Is your feature request related to a problem? Please describe.
It would be useful to add attributes to a RenderFragment. Similarly to how you can match any attributes with a catch-all dictionary.

### Describe the solution you'd like
In some of our code where we are porting over from a legacy webforms application, it would be nice to be able to define additional attributes that can be captured from a RenderFragment tag definition. This would be more specific than the top-level capture of additional attributes on the main control tag.

For example, imagine a definition like the following:

```

My Header content


My Footer Content

```

The solution might be an addition to the Parameter attribute, for example:

```
[Parameter(CaptureUnmatchedValues = true)]
public Dictionary ControlAttributes { get; set; }

[Parameter(CaptureUnmatchedValues = true, Name="Header")]
public Dictionary HeaderAttributes { get; set; }

[Parameter(CaptureUnmatchedValues = true, Name="Footer")]
public Dictionary FooterAttributes { get; set; }
```

### Additional context
Think of something like a dialog window where you might define a header, a footer, and then anything else could go into the content.