Introduce EF.Functions.DateDiffBig* overloads returning long

What problem are you trying to solve?

```
Microsoft.Data.SqlClient.SqlException (0x80131904): The datediff function resulted in an overflow. The number of dateparts separating two date/time instances is too large. Try to use datediff with a less precise datepart.
```

```cs
db.Bars.Where(x =>
EF.Functions.DateDiffMinute(x.PeriodStart, x.PeriodEnd) > validForMinutes
)
```

I can't use a less precise datepart so the solution is to use `DATEDIFF_BIG`.

```cs
db.Bars.Where(x =>
EF.Functions.DateDiffMinuteLong(x.PeriodStart, x.PeriodEnd) > validForMinutes
)
```

Am I missing something?