Introduce a ByteString model field

As models have to be JSON-serializable and byte
fields apparently don't fit in, let's introduce
a special model field type that could be used in
such situations.
Basic ByteString serialization tests added.

OAMG-4306

leapp/models/fields/__init__.py

@@ -1,3 +1,4 @@
+import base64
 import copy
 import datetime
 import json
@@ -239,6 +240,33 @@ def _model_type(self):
         return six.string_types + (six.binary_type,)
 
 
+class ByteString(BuiltinField):
+    """
+    ByteString field
+    """
+    @property
+    def _model_type(self):
+        # NOTE(ivasilev) Both string_types and binary_types are necessary here to pass value checks on
+        # serialization/deserialization. Serialized JSON-friendly model will have of a string type field (the base64
+        # ascii string) while deserialized model will have a binary type field.
+        return six.string_types + (six.binary_type,)
+
+    def _convert_to_model(self, value, name):
+        self._validate_model_value(value=value, name=name)
+        if not value:
+            return value
+
+        return base64.b64decode(value)
+
+    def _convert_from_model(self, value, name):
+        self._validate_model_value(value=value, name=name)
+        if not value:
+            return value
+
+        # NOTE(ivasilev) b64 encoding is always ascii, thus ascii decoding to get a string
+        return base64.b64encode(value).decode('ascii')
+
+
 class DateTime(BuiltinField):
     """
     DateTime field to handle datetime objects which are converted to the ISO format and parsed back from there

tests/scripts/test_serialization.py

@@ -41,6 +41,11 @@ class WithNestedListModel(Model):
     items = fields.List(fields.Model(BasicModel))
 
 
+class WithByteStringModel(Model):
+    topic = ModelTestTopic
+    message = fields.ByteString()
+
+
 class AllFieldTypesModel(Model):
     topic = ModelTestTopic
     float_field = fields.Float(default=3.14)
@@ -74,6 +79,12 @@ def test_basic_model():
     assert m.message == m2.message
 
 
+def test_bytestring_model():
+    m = WithByteStringModel(message=b'\xf3\xcf\xcf\xc2\xdd\xc5\xce\xc9\xc5')
+    m2 = WithByteStringModel.create(m.dump())
+    assert m.message == m2.message
+
+
 def test_string_list_model():
     m = WithStringListModel(messages=['Some message'])
     m2 = WithStringListModel.create(m.dump())