Feature-Fluid Compute Pricing

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**Description**
When calling 3rd party services such as LLMs, the lambda sits blocked on the network read, which customers are charged for, while that CPU is very likely doing other useful work. This at least is my understanding, and this doesn't seem right.

Vercel has added a variant of this called 'Fluid Compute Pricing' (on top of Lambda??): https://vercel.com/docs/functions/usage-and-pricing

Lambda should natively support this! And better i'm sure.

**Additional context**
Some of the other issues might be related to this in theory so happy to see this closed if other use cases take on this fundamental shift;

https://github.com/aws/aws-lambda-roadmap/issues/57
https://github.com/aws/aws-lambda-roadmap/issues/16

Start by reviewing the linked AWS Lambda roadmap issues and the Vercel Fluid Compute Pricing documentation. The issue does not name files, tests, or an implementation entry point; first establish the intended pricing model, scope, and relationship to those existing issues. Done would require an accepted product and implementation direction, not a self-contained code change.

Written by the indexing model from the issue text.