SqlSyntaxExtensions should be an instance method to allow database specific overrides

[idseefeld]

The SqlSyntaxExtensions methods should be instance methods like all the other ISqlSyntaxProvider methods and not an extension method. As instance method the only public virtual string GetFieldName<TDto>(...) method could be overridden. This would also improve code consistency.

This change is not a breaking change. All references will work with an instance method too and to avoid breaking change issues, the interface provides a temporary implementation. ISqlSyntaxProvider has already a ToDo, that all implementations should be removed in v18.

I have already a branch in my fork: refactor-SqlSytaxExtensions-to-instance-methods to implement this.