Fix "Map omits required key" exception by comparing result order against modern field keys

Summary:
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

Test Plan:

• Have a custom field (with search: true) defined via http://phorge.localhost/config/edit/maniphest.custom-field-definitions/
• Have two tasks have the custom field set
• Go to http://phorge.localhost/maniphest/query/ , select the custom field (e.g. "Due Date (Reversed)") in the "Order By" field, and click the "Search" button. Iterate through the results via the "Next" button if needed

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T15631

Differential Revision: https://we.phorge.it/D25504