api: Reduce allocations in get_user_list

[GodTamIt]

This change cleans up some references vs. ownership usage to reduce the amount of unneeded allocation done in the query building.

[GodTamIt]

N.B.: taking an impl IntoIterator<Item = &str> instead of Vec<String> for fields may be a better API surface for users who may not always have ownership and/or want to clone. Especially since we don't really need a String or the Vec itself

[DarrenBaldwin07]

N.B.: taking an impl IntoIterator<Item = &str> instead of Vec<String> for fields may be a better API surface for users who may not always have ownership and/or want to clone. Especially since we don't really need a String or the Vec itself

I agree with this. Since everything in /models and /api is generated from openapi spec, the code is pretty mediocre.