How to list the supported services?

[tisonkun]

Due to packaging difference, a specific opendal lib can support different range of services, it would be great that we provide a reflect method to list supported services; for example:

pub fn supported_services() -> Vec<String> {
  // returns ["memory", "fs", "s3", "redis", ...]
}

I want this features because packaging can be complicated in Java binding and I'd like to have a log for supported services for debugging.

Also, it's possible that users want to fallback to other scheme if the preferred one is not available - although it can be done with handling exception thrown by a failed initialization, it's not optimal.