Add exists method on the kv object

[oleiade]

Users can now use exists to check if a key exists, before taking action.

import { openKv } from "k6/x/kv";

const kv = openKv();

export async function setup() {
    // Start with a clean state
    await kv.clear();
}
export default async function () {
    const abcExists = await kv.exists("a b c")
    if (!abcExists) {
      await kv.set("a b c", { "123": "baby you and me girl"});
    }

    await kv.delete("a b c");
}

[Copilot]

PR Overview

This pull request introduces a new "exists" method on the KV object to allow checking if a key already exists before performing further operations.

• Introduces a new Exists method in kv/kv.go that accesses BoltDB to determine key existence.
• Updates README.md to include usage examples and documentation for the new method.

Reviewed Changes

File	Description
kv/kv.go	Adds the Exists method to check key presence using a BoltDB view.
README.md	Updates the documentation to include usage details for the exists API.

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

Comments suppressed due to low confidence (1)

kv/kv.go:174

• Consider aligning the error message style for a missing bucket with the Get method to maintain consistency across the API.

if bucket == nil {