[#627] morph: Define partial interface of StaticClient type

There is a need to inherit some methods of `StaticClient` type. In order to
not inherit all method via type embedding we can group sub-set of methods
and inherit it.

Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>

pkg/morph/client/internal/static.go

@@ -0,0 +1,11 @@
+package internal
+
+import (
+	"github.com/nspcc-dev/neo-go/pkg/util"
+)
+
+// StaticClient groups client.StaticClient type interface methods which are needed to be inherited
+// by upper-level client instances.
+type StaticClient interface {
+	ContractAddress() util.Uint160
+}