Support configurable manifest names in Chisel

When running chisel cut with base-files_chisel included, Chisel generates a security manifest file. However, in multi-part builds, every part saves its manifest using the same default filename. This causes manifest files conflict, making it impossible to easily preserve or combine manifest data across multiple build parts.

Allow to configure manifest file name via a command line option and make craft-parts provide a sensible default.

Security Manifest generator should be able to pick up part manifests and combine them into a single rock/snap manifest.

Start at the chisel cut command and trace how the security manifest is generated when base-files_chisel is included. Check how multi-part builds currently choose the default manifest name, then run the relevant cut workflow to verify configurable names, a craft-parts default, and combined rock/snap manifest data.

Written by the indexing model from the issue text.