data.ref: versioning XML schemas of "factura electrónica"

Move the content of the `schemas-xml` directory into a subdirectory
named `2011_05_30`. This name represents the version of these XML
schemas according to the last update timestamp of the files in the set.

Extra:
New enum `cl_sii.base.constants.XmlSchemasVersionEnum` to define
the available XML schema versions

Author: ycouce-cdd

cl_sii/base/constants.py

@@ -3,6 +3,8 @@
 ================
 
 """
+import enum
+
 import pytz
 
 from cl_sii.libs.tz_utils import PytzTimezone
@@ -11,3 +13,19 @@
 TZ_CL_SANTIAGO: PytzTimezone = pytz.timezone('America/Santiago')
 
 SII_OFFICIAL_TZ = TZ_CL_SANTIAGO
+
+
+class XmlSchemasVersionEnum(enum.Enum):
+    """
+    Enum of "SII XML Schema Versions".
+    The version name is selected considering the last updated timestamp
+    of the files in the set
+
+    """
+
+    V2011_05_30 = '2011_05_30'
+
+    @classmethod
+    def latest(cls) -> "XmlSchemasVersionEnum":
+        """Reference to the latest version available"""
+        return cls.V2011_05_30