rpc: add method to test for subscription support (#25942)

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
    }

---------

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

rpc/handler.go

@@ -530,10 +530,7 @@ func (h *handler) handleCall(cp *callProc, msg *jsonrpcMessage) *jsonrpcMessage
 // handleSubscribe processes *_subscribe method calls.
 func (h *handler) handleSubscribe(cp *callProc, msg *jsonrpcMessage) *jsonrpcMessage {
 	if !h.allowSubscribe {
-		return msg.errorResponse(&internalServerError{
-			code:    errcodeNotificationsUnsupported,
-			message: ErrNotificationsUnsupported.Error(),
-		})
+		return msg.errorResponse(ErrNotificationsUnsupported)
 	}
 
 	// Subscription method name is first argument.