Support feteching T, (T, U), (T, U, P)..etc where they impl TryGetableMany

[MuhannadAlrusayni]

Resolve #180

This is not the best API for sure! but we can start with this. One blocker that I got was SelectorTrait::from_raw_query_result is a function, which doesn't help when I want to store some data into SelectGetableValue, so I used generic type C to work around this, but yeah we don't get very ergonomic API.

[MuhannadAlrusayni]

if we could convert SelectorTrait::from_raw_query_result to be a method instead of function, that would help us have way better API in this PR.

But I think this is breaking change API.

[billy1624]

Thanks!! @MuhannadAlrusayni

if we could convert SelectorTrait::from_raw_query_result to be a method instead of function, that would help us have way better API in this PR.

I think this is worth doing as this enable us to store metadata that is useful when performing from_raw_query_result. And it's potentially useful for downstream types that implement SelectorTrait.

[tyt2y3]

I think we can use sea_query::Iden to replace ToString.
I will make the changes.