providers/saml: big cleanup, simplify base processor

add New fields for
 - assertion_valid_not_before
 - assertion_valid_not_on_or_after
 - session_valid_not_on_or_after
allow flexible time durations for these fields
fall back to Provider's ACS if none is specified in AuthNRequest

passbook/providers/saml/utils/time.py

@@ -0,0 +1,45 @@
+"""Time utilities"""
+import datetime
+
+from django.core.exceptions import ValidationError
+from django.utils.translation import gettext_lazy as _
+
+ALLOWED_KEYS = (
+    "days",
+    "seconds",
+    "microseconds",
+    "milliseconds",
+    "minutes",
+    "hours",
+    "weeks",
+)
+
+
+def timedelta_string_validator(value: str):
+    """Validator for Django that checks if value can be parsed with `timedelta_from_string`"""
+    try:
+        timedelta_from_string(value)
+    except ValueError as exc:
+        raise ValidationError(
+            _("%(value)s is not in the correct format of 'hours=3;minutes=1'."),
+            params={"value": value},
+        ) from exc
+
+
+def timedelta_from_string(expr: str) -> datetime.timedelta:
+    """Convert a string with the format of 'hours=1;minute=3;seconds=5' to a
+    `datetime.timedelta` Object with hours = 1, minutes = 3, seconds = 5"""
+    kwargs = {}
+    for duration_pair in expr.split(";"):
+        key, value = duration_pair.split("=")
+        if key.lower() not in ALLOWED_KEYS:
+            continue
+        kwargs[key.lower()] = float(value)
+    return datetime.timedelta(**kwargs)
+
+
+def get_time_string(delta: datetime.timedelta = None) -> str:
+    """Get Data formatted in SAML format"""
+    now = datetime.datetime.now()
+    final = now + delta
+    return final.strftime("%Y-%m-%dT%H:%M:%SZ")