Support DynamoDBHashKey and DynamoDBRangeKey attributes

[howcheng]

If an entity has property names that are different from the key column names, you are supposed to use the [Amazon.DynamoDBv2.DataModel.DynamoDBHashKey] attribute, but it looks like you don't support this yet in the EntityKeyGetter class. You can probably make a method to scan the entity's properties for these attributes.

[LCHarold]

Here's a method I'm using in a generic base class to get the hash key property name:

public static string GetHashKey<T>()
{
    return (typeof(T).GetProperties()
        .Where(p => p.GetCustomAttributes<DynamoDBHashKeyAttribute>()
        .Any())
        .Select(p => p.Name))
        .First();
}