orm: implement custom select field

[enghitalo]

Today, select fields is not customizable.

Suggestions

struct Module {
	id    int       [primary; sql: serial]
	a     string
	b     string
	c     string
}

//1
result := sql db {
    select (a, b) from Module where id == 1
}

//2
result := sql db {
    select a, b from Module where id == 1
}

//3
result := sql db {
    select "a b" from Module where id == 1
}

select_fields = "a b"
result := sql db {
    select select_fields from Module where id == 1
}

//4
result := sql db {
    select "a, b" from Module where id == 1
}

select_fields = "a, b"
result := sql db {
    select select_fields from Module where id == 1
}

//5
result := sql db {
    select ("a, b") from Module where id == 1
}

//6
mut select_fields := map[string]int{}

select_fields = {a:1, b:1}
result1 := sql db {
    select select_fields from Module where id == 1
}

select_fields = {c:-1}
result1 := sql db {
    select select_fields from Module where id == 1
}