ETag header is unavailable to the browser for pre-signed requests

[twick00]

Although S3Proxy responds with an included ETag header to multipart uploads, this header isn't seen by the browser because S3Proxy doesn't send Access-Control-Expose-Headers: Etag back with the browser.

If s3proxy.cors-allow-origins contains the origin we're replying to, we should include the Access-Control-Expose-Headers: Etag header as well. Same for if s3proxy.cors-allow-all=true is set.

[twick00]

@gaul I don't mind contributing to fix this but I could use your guidance on how to best fix it.

This is the simplest way to handle this:
#668

However, we could also include a new S3PROXY_CORS_EXPOSED_HEADER (s3proxy.cors-exposed-headers) property and allow the user to pass in the headers they want to access. I personally only care about ETag but I'm not sure if others might want other headers.