Support allowlisting via struct literal level comment rather than global config

[navijation]

Similar to #37, but at the level of struct literals rather than struct definitions.

The use case: ensuring that "type mapper" functions are properly updated when the destination type gets updated. For example

type InternalStatement struct {
	Balance   int
	StartDate time.Time
	EndDate   time.Time
}

type ExternalStatement struct {
	Balance   money.Amount `json:"balance"`
	StartDate string       `json:"start_date"`
	EndDate   string       `json:"end_date"`
}

func ToExternalStatement(statement *InternalStatement) *ExternalStatement {
	//exhaustruct:enforce
	return &ExternalStatement{
		Balance:   money.FromCents(statement.Balance),
		StartDate: formatMonthDay(statement.StartDate),
		EndDate:   formatMonthDay(statement.EndDate),
	}
}

If the external statement acquires a new field such as "overdue balance", the analyzer will catch if the mapper has not been updated to populate this new field. However, it is might not necessary for every external statement instantiation to require every field be populated.