Enable S3 plugin for the proxy

[bbockelm]

While the client has a generic plugin mechanism allowing any scheme (https://, root://, ...), the proxy server enumerates the permitted schemes one-by-one.

I'm tinkering with a s3:// plugin for the client which would allow us to unify our HTTP and S3 support into one library (instead of nearly-identical code, once for the server, once for the client). Adding a small patch that exposes s3:// as a supported protocol in the proxy.

[matyasselmeci]

Included in xrootd-5.8.4-1.3

[matyasselmeci]

@bbockelm is this ready for a PR to upstream?

[matyasselmeci]

This is included upstream as part of xrootd#2567, targeted for 6.0.0.