Support for generic Column indexes, and enum creation

[ToxicMushroom]

This change adds an option column_index_type to switch the current default usize column indexes to a generated enum.

• Omitting this annotation tells the code to fill in usize into the generic, essentially keeping all previous behaviour.
• Providing this option with a string != "usize" causes an enum to be generated and used.

(I dislike this current config type, I wanted to use an enum first but this crate can't export that).

This pr serves as a progress tracker currently.

[maccesch]

looking good!

[ToxicMushroom]

I added support for my extra changes in Synphonyte/leptos-struct-table#87 (Column ordering+visibility and the drag and drop support)