feat: allow fetching vectors using a prefix

[heyjorgedev]

This PR introduces a new way to fetch vectors using the SDK. Before you could fetch directly records from our vector database using their ids, now you can fetch them by referencing their id prefix.

<?php

use Upstash\Vector\Index;
use Upstash\Vector\VectorFetchByPrefix;

$index = new Index(...);

$results = $index->fetch(new VectorFetchByPrefix(
    prefix: 'user:',
    includeVectors: true,
));

[linear]

DX-1667 PHP: Support fetch with id prefix for Vector PHP