This adds two ways to check for subscription support. First, one can now check
whether the transport method (HTTP/WS/etc.) is capable of subscriptions using
the new Client.SupportsSubscriptions method.

Second, the error returned by Subscribe can now reliably be tested using this
pattern:
    
    sub, err := client.Subscribe(...)
    if errors.Is(err, rpc.ErrNotificationsUnsupported) {
        // no subscription support
    }

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Co-authored-by: Felix Lange <fjl@twurst.com>