Add support for static columns with StaticColumnAttribute and cm.AsStatic()

Possibility to specify static columns in entities...

    [Table(Name = "Items", CaseSensitive = false)]
    public class LinqDecoratedEntityWithStaticField
    {
        [PartitionKey]
        public int Key { get; set; }

        [StaticColumn]
        public string KeyName { get; set; }

        [ClusteringKey(1)]
        public int ItemId { get; set; }

        public decimal Value { get; set; }
    }

... and ColumnMaps

    var typeDefinition = new Map<AllTypesDecorated>()
        .PartitionKey(t => t.UuidValue)
        .Column(t => t.UuidValue, cm => cm.WithName("id"))
        .Column(t => t.StringValue, cm => cm.WithName("name").AsStatic())