feat: add support for uploading images via blazor method or controller.

[dandeto]

This may resolve #10, but this might not be exactly what you had in mind. Regardless, I added the ability to upload images via a Blazor method, or a controller action. After uploading, the images get inserted as an img tag with the src attribute set to the path of the uploaded image. This is something I need for my project, so I went ahead and fixed it myself. The base64 encoding behavior continues to be enabled by default - this server-side uploading feature must be configured by the user.

In the MudHtmlEditor.razor.cs file, I added three new parameters and one new method. Also, I added a settings object to group the existing placeholder setting with the new ones I added before they get passed via interop to the JS side of the component.

In MudHtmlEditor.razor.js, I added uploadImageHandler - a custom handler for the (unfortunately undocumented) quill uploader module - and a supporting upload function which does the actual upload to the server.

In the remaining files, I prepared an example controller method and Blazor method that will upload the image to the wwwroot/images folder on the server, and return a url like images/image.png as the response for the JS to pick back up so it can insert that url as part of the resulting img tag.