Add in_group and has_attribute policy functions

This PR adds two policy functions.

1. in_group checks whether an Abbey user's identity is within a group

2. has_attribute checks whether an Abbey user has a custom attribute with
a given value

src/abbey/functions/has_attribute.rego

@@ -0,0 +1,5 @@
+import future.keywords.if
+
+has_attribute(name, value) := true if {
+	data.system.abbey.identities.directory_sync_users.custom_attributes[name] == value
+}

src/abbey/functions/in_group.rego

@@ -0,0 +1,7 @@
+import future.keywords.if
+import future.keywords.in
+
+in_group(group_name) := true if {
+	some group in data.system.abbey.group_memberships
+	group == group_name
+}