Expose RepositoryVersionSerializer in the plugin API

In pulpcore 3.88 it was added a feature that would the return value
of task functions, and those return must be serializable or None.

In 3.100 that would be required by pulpcore, so plugins that wante
to adapt before might use the RepositoryVersionSerializer. In this case,
regardless of whether pulpcore >=3.88 or <3.88 is used, the plugin must
be able to import the serializer or it will throw a runtime import error.

(cherry picked from commit 20f3b7a)

Author: pedro-psb

pulpcore/plugin/serializers/__init__.py

@@ -31,6 +31,7 @@
     RepositorySerializer,
     RepositorySyncURLSerializer,
     RepositoryVersionRelatedField,
+    RepositoryVersionSerializer,
     SingleArtifactContentSerializer,
     SingleContentArtifactField,
     TaskGroupOperationResponseSerializer,
@@ -78,6 +79,7 @@
     "RepositorySerializer",
     "RepositorySyncURLSerializer",
     "RepositoryVersionRelatedField",
+    "RepositoryVersionSerializer",
     "SingleArtifactContentSerializer",
     "SingleContentArtifactField",
     "TaskGroupOperationResponseSerializer",