♻️(models) set missing default values for xAPI activity types

Some activity types declared with Pydantic were missing default values for types
field which forces library users to declare the type value when model is
instantiated. However, as these values are constants in xAPI semantics, we do
not want to request this from lib users.

src/ralph/models/xapi/concepts/activity_types/acrossx_profile.py

@@ -47,7 +47,9 @@ class WebpageActivityDefinition(BaseXapiActivityDefinition):
             `https://w3id.org/xapi/acrossx/activities/webpage`.
     """
 
-    type: Literal["https://w3id.org/xapi/acrossx/activities/webpage"]
+    type: Literal["https://w3id.org/xapi/acrossx/activities/webpage"] = (
+        "https://w3id.org/xapi/acrossx/activities/webpage"
+    )
 
 
 class WebpageActivity(BaseXapiActivity):

src/ralph/models/xapi/concepts/activity_types/activity_streams_vocabulary.py

@@ -45,7 +45,7 @@ class FileActivityDefinition(BaseXapiActivityDefinition):
        type (str): Consists of the value `http://activitystrea.ms/file`.
     """
 
-    type: Literal["http://activitystrea.ms/file"]
+    type: Literal["http://activitystrea.ms/file"] = "http://activitystrea.ms/file"
 
 
 class FileActivity(BaseXapiActivity):

src/ralph/models/xapi/concepts/activity_types/audio.py

@@ -21,7 +21,9 @@ class AudioActivityDefinition(BaseXapiActivityDefinition):
             `https://w3id.org/xapi/audio/activity-type/audio`.
     """
 
-    type: Literal["https://w3id.org/xapi/audio/activity-type/audio"]
+    type: Literal["https://w3id.org/xapi/audio/activity-type/audio"] = (
+        "https://w3id.org/xapi/audio/activity-type/audio"
+    )
 
 
 class AudioActivity(BaseXapiActivity):

src/ralph/models/xapi/concepts/activity_types/tincan_vocabulary.py

@@ -21,7 +21,9 @@ class DocumentActivityDefinition(BaseXapiActivityDefinition):
             `http://id.tincanapi.com/activitytype/document`.
     """
 
-    type: Literal["http://id.tincanapi.com/activitytype/document"]
+    type: Literal["http://id.tincanapi.com/activitytype/document"] = (
+        "http://id.tincanapi.com/activitytype/document"
+    )
 
 
 class DocumentActivity(BaseXapiActivity):
@@ -45,7 +47,9 @@ class WebinarActivityDefinition(BaseXapiActivityDefinition):
             `http://id.tincanapi.com/activitytype/webinar`.
     """
 
-    type: Literal["http://id.tincanapi.com/activitytype/webinar"]
+    type: Literal["http://id.tincanapi.com/activitytype/webinar"] = (
+        "http://id.tincanapi.com/activitytype/webinar"
+    )
 
 
 class WebinarActivity(BaseXapiActivity):