feat: add KV storage adapters

[r1tsuu]

Adds KV storage support to Payload Local API with an adapter pattern, includes 3 adapters. We'll use it for the Realtime API.
You can access it via payload.kv:

interface KVAdapter {
  /**
   * Clears all entries in the store.
   * @returns A promise that resolves once the store is cleared.
   */
  clear(): Promise<void>

  /**
   * Deletes a value from the store by its key.
   * @param key - The key to delete.
   * @returns A promise that resolves once the key is deleted.
   */
  delete(key: string): Promise<void>

  /**
   * Retrieves a value from the store by its key.
   * @param key - The key to look up.
   * @returns A promise that resolves to the value, or `null` if not found.
   */
  get(key: string): Promise<KVStoreValue | null>

  /**
   * Checks if a key exists in the store.
   * @param key - The key to check.
   * @returns A promise that resolves to `true` if the key exists, otherwise `false`.
   */
  has(key: string): Promise<boolean>

  /**
   * Retrieves all the keys in the store.
   * @returns A promise that resolves to an array of keys.
   */
  keys(): Promise<string[]>

  /**
   * Sets a value in the store with the given key.
   * @param key - The key to associate with the value.
   * @param value - The value to store.
   * @returns A promise that resolves once the value is stored.
   */
  set(key: string, value: KVStoreValue): Promise<void>
}

To configure the adapter you can use the kv property of the root config.

buildConfig({
  kv: adapter()
})

Database KV adapter (default)

No need to configure, as Payload uses it by default. It generates new collection payload-kv and uses the current database adapter to access it. The collection is hidden in the admin panel and access locked.
If you want to override the generated collection:

import { databaseKVAdapter } from 'payload'

buildConfig({
  kv: databaseKVAdapter({
    kvCollectionOverrides: {
      slug: 'custom-kv',
      ...(process.env.DEBUG === 'true' && {
        admin: { hidden: false },
        access: {},
      }),
    },
  }),
})

In Memory KV adapter

Simple and very fast storage using memory. Don't use it on Vercel / multiple instances or if you need data persistence.

import { inMemoryKVAdapter } from 'payload'

buildConfig({
  kv: inMemoryKVAdapter(),
})

Redis KV Adapter

Uses Redis. Probably the best option as it's faster than database, persistent and works with Vercel / multiple instances, but requires additional setup

pnpm add @payloadcms/kv-redis

import { redisKVAdapter } from '@payloadcms/kv-redis'

buildConfig({
  kv: redisKVAdapter({
    keyPrefix: "custom-prefix:", // defaults to 'payload-kv:'
    redisURL: "redis://127.0.0.1:6379" // defaults to process.env.REDIS_URL (Vercel generates this variable for you if you connect a project to Redis)
  }),
})