Allow async method for HeaderPropagationOptions.ValueFilter

## Background and Motivation

Consider following code:

``` csharp

builder.Services.AddHttpClient()
.AddHeaderPropagation(options =>
{
options.Headers.Add("Authorization", context =>
{
// await here
var access_token = context.HttpContext.GetTokenAsync("access_token").GetAwaiter().GetResult();

return access_token ?? StringValues.Empty;
});
});

```
## Proposed API

Add property `AsyncValueFilter` for HeaderPropagationEntry.

```diff
namespace Microsoft.AspNetCore.HeaderPropagation;

public class HeaderPropagationEntry
{
+ public Func>? AsyncValueFilter { get; }
}

public sealed class HeaderPropagationEntryCollection : Collection
{
+ public void Add(string headerName, Func> valueFilter);
}
```

## Usage Examples

```csharp
builder.Services.AddHttpClient()
.AddHeaderPropagation(options =>
{
options.Headers.Add("Authorization", async context =>
{
// await here
var access_token = await context.HttpContext.GetTokenAsync("access_token");

return access_token ?? StringValues.Empty;
});
});
```