Summary

Adds OverrideCacheTTL() function to allow setting custom TTL for cache entries based on the data being cached.

Use Case

Enable different TTLs for positive vs negative cache entries:

• Positive entries (data found): longer TTL (e.g., 5 minutes)
• Negative entries (data not found): shorter TTL (e.g., 30 seconds)

This prevents stale negative entries when data becomes available shortly after an initial miss.

Usage

val, err := cache.Get(ctx, 5*time.Minute, key, func(ctx context.Context, key string) (string, error) {
    user, err := db.GetByID(ctx, userID)
    if err == sql.ErrNoRows {
        rueidisaside.OverrideCacheTTL(ctx, 30*time.Second)
        return "NOT_FOUND", nil  // cached for 30 seconds
    }
    return user, nil  // cached for 5 minutes
})

Closes #921