InputFile - Easy setup of uploaded files

Description

This PR relates directly to #317 (closes #317) and should give an easy setup for InputFile. In its current state there is no easy way of setting up an InputFile. This PR introduces two methods which makes it possible.

1. AddInputFileSupport to setup the JavaScript part
2. UploadFilesAsync to configure the files which will be returned by InputFile and also directly invoking the OnChange event.

Implementation

The implementation is straight forward. We just introduce a bunch of new types which are test doubles of the ones from the framework itself. I decided to go with an extension for IRenderedComponent<InputFile> because users can have multiple InputFiles in their code and they are used to call cut.Find("..."); anyway.

Usage

A typical test would look like this:

[Fact]
public void ShouldUploadFile()
{
    var cut = RenderComponent<ComponentWithInputFile>();
    var uploadFile = InputFileContent.CreateFromText("Hello World");

    cut.FindComponent<InputFile>().UploadFiles(uploadFile);

What is open? / discussion

Here are some points I want to discuss:

• Right now, we have to call two methods to make it work. Without AddInputFileSupport we will get JavaScript Interop errors that the call was not set up in strict mode. I didn't want to mock that call (Blazor._internal.InputFile.init) by default. But we could. Via default we setup the JS call and let the user opt-out via an extension method. That might make more sense, because on average the user does not want to setup InputFile stuff on their own.
• We might want to have that call synchronous to fit better to the current API. Simply speaking we can just await the internal call synchronously.
• The used InputFileContent is always byte[]. I played around with two versions: string and byte[] and went with byte[] because it makes our code easier and pushes the responsibility to the caller instead of us. Text / encoding can be very hard. And especially when people want to upload images or so, byte[] seems the appropriate way.
• It is hard to create a InputFile test in any of our test assemblies because that type was introduced with net5.0. Either we build one from scratch via BuildRenderTree(RenderTreeBuilder builder). If you have any tips, let me know.

Alternatives

I played around with the idea to replace or a given InputFile via our ComponentFactories with a test double. We might make this work as well, but it doesn't seem that much nice when it comes down to implementation details.

Info

This PR is only draft atm and meant as a base for discussions. Once we are aligned on the API design, xmldoc and tests will follow.