Function get_random_key_values_prefix

pub fn get_random_key_values_prefix<R: Rng>(
    rng: &mut R,
    key_prefix: Vec<u8>,
    len_key: usize,
    len_value: usize,
    num_entries: usize,
    key_byte_upper_limit: u8,
) -> Vec<(Vec<u8>, Vec<u8>)>

Takes a random number generator, a key_prefix and generates pairs (key, value) with key obtained by appending len_key random bytes to key_prefix and value obtained by creating a vector with len_value random bytes. We return n such (key, value) pairs which are all distinct.