Make forge app max payload size configuable

### Description

The fastify payload size has just been increased to 10mb from 5mb

This requires a code change, but would be better set as configuration option.

### Which customers would this be available to

Everyone - CE/Starter/Team/Enterprise

### Have you provided an initial effort estimate for this issue?

I have provided an initial effort estimate

Start by locating the Fastify payload-size setting that was changed from 5 MB to 10 MB. Trace how application configuration is defined and applied, then verify that the payload limit can be changed through configuration for all customer editions and add or update the relevant tests if they exist.

Written by the indexing model from the issue text.