Add ability to customize mapping convention

[stasberkov]

When configuring C# class fields to table column mapping I have to specify cm.WithName for every column. It would be great to allow automatic PascalCase to snake_case field name conversion. Please add an ability to write such converters (I can implement converter by myself).

[stasberkov]

I Implemented something like this to customize property names:

public static class MappingExtensions {
    public static Map<T> AllColumnsSnakeCase<T>(this Map<T> map) {
        var props = typeof(T)
            .GetProperties(BindingFlags.Public | BindingFlags.Instance)
            .Where(p => p.CanRead && p.CanWrite && p.GetIndexParameters().Length == 0)
            ;

        foreach (var prop in props) {
            var param = Expression.Parameter(typeof(T), "x");
            var access = Expression.Property(param, prop);
            var funcType = typeof(Func<,>).MakeGenericType(typeof(T), prop.PropertyType);
            dynamic fieldAccessExpression = Expression.Lambda(funcType, access, param);
            Action<ColumnMap> columnMapAction = cm => cm.WithName(ToSnakeCase(prop.Name));
            map.Column(fieldAccessExpression, columnMapAction);
        }

        return map;
    }

    public static string ToSnakeCase(string input) {
        if (string.IsNullOrEmpty(input))
            return input;

        var sb = new StringBuilder();
        sb.Append(char.ToLowerInvariant(input[0]));
        for (int i = 1; i < input.Length; i++) {
            char c = input[i];
            if (char.IsUpper(c)) {
                sb.Append('_');
                sb.Append(char.ToLowerInvariant(c));
            } else {
                sb.Append(c);
            }
        }
        return sb.ToString();
    }
}

and use it like

        For<TableOne>()
            .TableName(MappingExtensions.ToSnakeCase(nameof(TableOne)))
            .PartitionKey(x => x.SomeKey)
            .ClusteringKey(x => x.OtherKey)
            .AllColumnsSnakeCase()
            ;