Sorting a Maniphest search query by custom fields throws a "Map returned omits required key" exception.
The isCustomFieldOrderKey() check still tested against legacy field key format (for example [custom:]std:maniphest:deadline.due) while the code passes modern field key format (for example custom.deadline.due).
After fixing this, PhutilTypeSpec::checkMap() throws an exception when a non-optional (extra) key $column is in $columns but not in the array of type parameters below to check against:
"Got unexpected parameters: customfield, customfield.index.table, customfield.index.key"
Thus add optional types for customfields in buildPagingClauseFromMultipleColumns() to allow them instead of throwing another exception.
Closes T15631