[Feature Request] Request to support S3 for Image Helper

[hzj629206]

Currently the Image Helper supports image uploading to imgur, sm.ms, and qiniu.

Nowadays, there are a lot of S3 compatible Cloud Object Storage services. It's a good choice to store images in Object Storage.

So I request this plugin to support S3 uploading.

Refer to https://github.com/imlinhanchao/vsc-markdown-image/blob/master/src/lib/s3.ts