Add support for default parameter values in ViewComponent tag helper

[captainsafia]

Addresses #5011.

To implement support for optional parameters, we largely needed to do two things:

• Update the ViewComponentTagHelperDescriptorFactory that is responsible for constructing VCTHD objects to not produce the RequiredAttributeDescriptors for parameters that have a default value provided.
• Update the generated code for VCTHs to produce an arguments object to the InvokeCoreAsync method that contains only the provided parameters

Updates to the VCTH descriptor factory

The VCTH descriptor contains a RequiredAttributeDescriptor for every parameter to the InvokeAsync method on the VC. When a parameter has a default value provided, we don't need to configure the parameter as a RequiredAttribute on the VCTH descriptor.

With this change in place, instances of the VCTH that don't contain the required attribute will still match with the VCTH. For example, the following VC:

public class FooViewComponent : ViewComponent
{
    public IViewResult InvokeAsync(string foo, string bar = "default value")
    {
        return $"{foo} : {bar}";
    }
}

Would produce a VCTH descriptor that looks like this:

- ViewComponentTagHelper
  - RequiredAttributeDescriptor(propertyName = "foo")
  - BoundAttributeDescriptor(propertyName = "foo")
  - BoundAttributeDescriptor(propertyName = "bar")

And will match VCTH instances like:

<vc:foo foo="a-foo-value"></vc:foo>
<vc:foo foo="a-foo-value" bar="a-bar-value"></vc:foo>

But not VCTH instances that looks like:

<vc:foo bar="a-bar-value"></vc:foo>
<vc:foo></vc:foo>

All this can happen with very few changes to the tag helper matching logic that already exists.

Update the generated code for VCTHs

Presently, whenever a VCTH is encountered, generated code is produced that:

• Writes a class representing the view component which contains properties for each parameter provided to the InvokeAsync method and a ProcessAsync method that passes the class properties as arguments to InvokeAsync
• Instantiates an instance of the class above and sets class properties with their associated attributes

Even if the VCTH is matched to the VC successfully as a result of the changes above, the generated code will be inaccurate because null values will be passed for parameters that have default values. Instead, we want to construct an arguments object that only contains the parameters that were provided as attributes to the VCTH instance.

We do this by adding a new ProcessInvokeAsyncArgs that evaluates if each of the parameters was provided as an attribute to the VCTH instance and if so, sets its value in the arguments object.

private Dictionary<string, object> ProcessInvokeAsyncArgs(Microsoft.AspNetCore.Razor.TagHelpers.TagHelperContext __context)
{
    Dictionary<string, object> args = new();
    if (__context.AllAttributes.ContainsName("first-name"))
    {
        args[nameof(firstName)] = firstName;
    }
    return args;
}

The net effect is that we only pass set parameters as arguments to the method. For parameters that aren't passed as arguments, the default value will be used.

[NTaylorMullen]

You'll also need to update two Json Converters:

• https://github.com/dotnet/aspnetcore/blob/1a2b3260c6161ae9b7f639de228a6eb0488a1075/src/Shared/RazorShared/TagHelperDescriptorJsonConverter.cs
• https://github.com/dotnet/aspnetcore-tooling/blob/main/src/Razor/src/Microsoft.CodeAnalysis.Razor.Workspaces/Serialization/TagHelperDescriptorJsonConverter.cs

TagHelperDescriptoir's are super performance sensitive so we had to optimize their serialization as much as possible 😄

[NTaylorMullen]

Assuming there's no issues in the runtime utilizing this it looks legit! Love the simplicity!

[NTaylorMullen]

Looks beautiful!! 👏