feat(issues): Allow resetting due date, epic, milestone, and weight from issues.

With the vanilla JSON encoding rules, it is impossible to reset some issue fields, such as weight, because with omitempty encoding/json will never emit a null value.

This change adds dedicated booleans to indicate that a field should be reset. This maintains backwards compatibility and clearly expresses the intent. The fields use the Reset prefix to align with existing functionality in the issues API, specifically ResetTimeEstimate and ResetTimeSpent.

Closes: #2110 (closed)