✨ Add Redis cache support

[Paillat-dev]

This PR adds Redis cache support to Botkit:

• Add Redis cache configuration options
• Add fallback to memory cache with warning if Redis config is missing
• Add documentation for cache configuration
• Update config.example.yaml with cache examples

The cache can now be configured in config.yml:

cache:
  type: "redis"
  redis:
    host: "localhost"
    port: 6379
    db: 0
    password: "optional"
    username: "optional"
    ssl: false

Or using environment variables:

BOTKIT__cache__type=redis
BOTKIT__cache__redis__host=redis.example.com