Blazor Server RenderBatchWriter should stop defensive copies of RenderTreeFrame on Write method

In .NET 5, `RenderTreeFrame` stopped being `readonly` in order to support some performance optimizations focused on WebAssembly. Generally this did not have any drawbacks because:

* The mutability is `internal`. We didn't make it publicly mutable.
* The framework always passes around `RenderTreeFrame` either with `ref` or with no modifier
* Or in the case of `RenderTreeDiffBuilder.NextSiblingIndex`, it does use `in` but it doesn't make any method calls or property accesses inside it (we probably should change this to use `ref` to have the behavior be more obvious).

However there remains one other case: `RenderBatchWriter`'s `Write` method:

```cs
void Write(in RenderTreeFrame frame)
{
....
}
```

Inside there, it performs lots of property reads. If this [blog post about perf implications of `in`](https://devblogs.microsoft.com/premier-developer/the-in-modifier-and-the-readonly-structs-in-c/) is correct, that means there will be lots of defensive copies happening.

To fix this, we could either remove the `in` modifier and just have a single copy made for each call to `Write`, or we could replace `in` with `ref` and have no copies made.

I don't yet have any specific measurement of the perf effects. We should at least try to measure this before committing to a specific resolution. It might not really have any observable real-world effects because `RenderBatchWriter` is so fast anyway relative to other things that are going on, such as transmitting render batches over the network.