Generate select_as and save_as attributes for custom column types.

[lucasmerlin]

PR Info

• Fixes sea-orm-cli generate entity migration select_as, save_as support #1643

New Features

• Generate select_as and save_as attributes to automatically cast custom types
  Before, using custom types in postgres resulted in a error like this:

DatabaseError occurred: Query Error: error occurred while decoding column "email": mismatched types; Rust type `core::option::Option<alloc::string::String>` (as SQL type `TEXT`) is not compatible with SQL type `citext`

Now it will be casted correctly.

I am not sure what the implications of adding select_as and save_as are for other database backends, since I believe this is only needed for postgres? So maybe it will need to be made configurable or only be enabled for postgres. Some feedback on this would be appreciated.

I've tested these changes in my codebase where it works great.

Breaking Changes

• Custom types will now have the select_as and save_as attributes generated. But since they wouldn't have been usable before, at least with postgres, I'm not sure if this is a breaking change.