[#504] morph/client: Add more global config value getters

Including:
- typo fix for `StringFromStackItem` error msg
- EigenTrust alpha getter.
- renaming local var in reading uin64 values
from global cfg function

Signed-off-by: Pavel Karpy <carpawell@nspcc.ru>

pkg/morph/client/netmap/config.go

@@ -59,3 +59,7 @@ func (c *Client) Config(args ConfigArgs, assert func(stackitem.Item) (interface{
 func IntegerAssert(item stackitem.Item) (interface{}, error) {
 	return client.IntFromStackItem(item)
 }
+
+func StringAssert(item stackitem.Item) (interface{}, error) {
+	return client.StringFromStackItem(item)
+}