Improve design to make it extensible to allow adding a new DB backend

[barsdeveloper]

Hello, I am interested in the Sea ecosystem especially the combination of query and orm but I want to use a DBMS which is not supported (DuckDB). As they are currently designed, these libraries make it so that it's impossible to introduce a new backend outside of this crate as discussed already here: SeaQL/sea-orm#2369
Enum based polymorphism was mentioned in that discussion but for example but there is more, for example in sea-query you cannot inspect the content of a statement because all the fields are pub(crate) instead of having proper pub methods to inspect their content.

pub struct SelectStatement {
    pub(crate) distinct: Option<SelectDistinct>,
    pub(crate) selects: Vec<SelectExpr>,
    // ...
} 

I think it would be very beneficial to spend some time refactoring the code to support it. I would like to start from sea-query (because I think it can be isolated better) to give it a try but I only want to do so if there would be interest from you (mantainers) to support it: give some feedback and guidance and accept a PR.

There is also a new DB backend work in progress: https://www.sea-ql.org/SeaORM-X/
I expect this to be a paid feature (not sure) and would like to get more information about how do you plan to integrate it. Will you have a separate code base from the open source version that adds just the bits to support SQL Server? Or what I am suggesting is already being worked on?