Support uploading screenshots

Hi,

This might be wrong place to open this issue, but I'm not sure here or https://github.com/JetBrains/gradle-intellij-plugin for this issue.

I want to upload screenshots of the plugins using gradle-intellij-plugin.

## What I did

I add a method for uploading screenshots

```kotlin
@Multipart
@POST("/plugin/uploadScreenshot")
fun uploadScreenshot(@Part("userName") username: TypedString, @Part("password") password: TypedString,
@Part("pluginId") pluginId: TypedString, @Part("channel") channel: TypedString?,
@Part("file") file: TypedFile): Response // I don't think parameter name `file` is correct. It should be `files[0]`
```

This gave me 400 error saying `Parameter csrfToken must be defined`.

Here is the source code. https://github.com/shiraji/plugin-repository-rest-client/tree/upload_screenshots

## Question

Could you tell me how I can upload screenshots without csrf token? like what uploading plugin module did.

Thanks in advance.