[#310] *: Implement string converters for enumerations

Implement `String` / `FromString` method pair in all levels of enum
definitions. From now `String()` returns canonical protojson-compatible
values.

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

v2/refs/string.go

@@ -0,0 +1,26 @@
+package refs
+
+import (
+	refs "github.com/nspcc-dev/neofs-api-go/v2/refs/grpc"
+)
+
+// String returns string representation of ChecksumType.
+func (t ChecksumType) String() string {
+	return ChecksumTypeToGRPC(t).String()
+}
+
+// FromString parses ChecksumType from a string representation.
+// It is a reverse action to String().
+//
+// Returns true if s was parsed successfully.
+func (t *ChecksumType) FromString(s string) bool {
+	var g refs.ChecksumType
+
+	ok := g.FromString(s)
+
+	if ok {
+		*t = ChecksumTypeFromGRPC(g)
+	}
+
+	return ok
+}