chore: Let Inserter accept &String and &Vec<u8>

[yutannihilation]

Currently, this doesn't work because the trait requires it to be &str, not &String. Just using .as_str() fixes the problem, but it's nice if insert() accept it as it is.

let value: String = some_function();
flat_vector.insert(index, &value);

I thought impl<T: AsRef<str>> Inserter<T> for FlatVector { works, but it's not allowed by the compiler. So, I added impls for &String and &Vec<u8> individually.

error[E0119]: conflicting implementations of trait `vector::Inserter<CString>` for type `vector::FlatVector`
   --> crates\duckdb\src\core\vector.rs:138:1
    |
130 | impl Inserter<CString> for FlatVector {
    | ------------------------------------- first implementation here
...
138 | impl<T: AsRef<str>> Inserter<T> for FlatVector {
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `vector::FlatVector`
    |
    = note: upstream crates may add a new impl of trait `std::convert::AsRef<str>` for type `std::ffi::CString` in future versions