CSHARP-986 LINQ support for writetime() function

[bastianeicher]

This adds support for LINQ queries like:

var result = myTable
    .Where(x => x.Id == "abc")
    .Select(x => new {WriteTime = CqlFunction.WriteTime(x.MyColumn)})
    .Execute();

They will be transformed to CQL like this:

SELECT WRITETIME(MyColumn) FROM my_table;

[joao-r-reis]

Hi, thanks for your contribution. Have you signed our CLA?

[bastianeicher]

Hi, thanks for your contribution. Have you signed our CLA?

Yes. :)

[joao-r-reis]

justfyi: C# Driver 3.19.0 is now GA (which includes this feature)

[bastianeicher]

Hello @joao-r-reis,

thank you very much for merging and releasing this change so swiftly! :)

Unfortunately, I overlooked something and chose the wrong return type for the new method. :( I've created a follow-up PR to fix this: #581