Programmatic file download support for Hilla endpoints

[platosha]

Add support for returning a Spring Resource result from Hilla endpoints. The files should be downloaded using an appropriate HTTP MIME type.

Example endpoint:

@BrowserCallable 
class DownloadEndpoint {
  public Resource loadFile(String fileId) {
    //...
  }
}

TypeScript usage:

const file = await DownloadEndpoint.loadFile("myFile.txt");
const url = URL.createObjectUrl(file);

[Legioth]

Note the point of this API would be to give TypeScript access to the actual bytes of the file. The bytes start being downloaded when the endpoint method is run.

In many cases, you instead want to create a link that the user can click to start a "regular" download. This requires a different mechanism since that link would trigger a GET request which also means that e.g. access control would have to handled without relying on custom request headers.