Create appup transform to set post_purge to soft

[Neylix]

Description

Create new module Archethic.Release.TransformPurge.
This module is called during the creation of an appup from distillery.
It is configured in rel/config.exs

Type of change

• New feature (non-breaking change which adds functionality)

How Has This Been Tested?

Create a release on testnet branch using ./script/test-script.sh -i
Switch to PR branch, change mix.exs version and generate an appup using mix distillery.gen.appup --app archethic
The generated appup should have brutal_purge and soft_purge in each load_module or update instructions.
Generate the upgrade release using ./script/test-release.sh -p it should take the generated appup as it valid (not create a new one, visible in log)

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules