Introducing Active CPU pricing for Fluid compute

What this post added

Introduces Active CPU pricing for Fluid compute, a new pricing model that charges for CPU only when code is actively executing. This model aims to further optimize costs for I/O bound workloads with high idle times, such as AI inference. The pricing is based on three metrics: Active CPU (milliseconds of vCPU usage), Provisioned Memory (GB-hours), and Invocations. This builds upon the existing Fluid compute model which intelligently orchestrates compute across invocations to eliminate cold starts and reuse idle time.