Feature/insert function values

[tomasfabian]

This PR extends the library with enabling custom KSQL functions in the following manner:

var insertValues = new InsertValues<Article>(new Article())
  .WithValue(c => c.Release_Date, FormatTimestamp(FROM_UNIXTIME(UnixTimestamp()), "yyyy"));

var insertStatement = ClassUnderTest.ToInsertStatement(insertValues);

[KSqlFunction]
public static string FormatTimestamp(long input, string format) => throw new NotSupportedException();

[KSqlFunction]
public static long FROM_UNIXTIME(long milliseconds) => throw new NotSupportedException();

[KSqlFunction]
public static long UnixTimestamp() => throw new NotSupportedException();

private struct Article
{
  [IgnoreByInserts]
  public long RowTime { get; set; }
  public string Title { get; set; }
  [Key]
  public int Id { get; set; }
  public string Release_Date { get; set; }
}