rpcclient: add Internal client for direct RPC server calls

It has a special `requestF` and a special initialization function, but other
than that it's an absolutely regular WSClient. Can be used to call, can be
used to subscribe. Fixes #2909.

pkg/rpcclient/local_test.go

@@ -0,0 +1,18 @@
+package rpcclient
+
+import (
+	"context"
+	"testing"
+
+	"github.com/nspcc-dev/neo-go/pkg/neorpc"
+	"github.com/stretchr/testify/require"
+)
+
+func TestInternalClientClose(t *testing.T) {
+	icl, err := NewInternal(context.TODO(), func(ctx context.Context, ch chan<- neorpc.Notification) func(*neorpc.Request) (*neorpc.Response, error) {
+		return nil
+	})
+	require.NoError(t, err)
+	icl.Close()
+	require.NoError(t, icl.GetError())
+}