Implement packages importing and freezing

This includes:

1. design, implement and document centralization and merging of the list of needed packages (e.g. topic branches can pull more packages in), if we decide to import only a subset of the Debian suites we’re interested in
2. set up and configure the chosen tools
3. do a first import
4. make it easy or automated to import foreign packages and freeze APT suites, whenever relevant during the dev and release cycle
5. update build process accordingly
6. update release process documentation accordingly

Subtasks will follow when we start working on it.

Blueprint: https://tails.boum.org/blueprint/freezable_APT_repository/

Feature Branch: feature/5926-freezable-APT-repository

Parent Task: #5926 (closed)

Subtasks

• #6296 (closed)
• #6299 (closed)
• #6297 (closed)
• #6303 (closed)
• #10748 (closed)
• #10749 (closed)
• #11444 (closed)
• #11446 (closed)

Original created by @intrigeri on 9489 (Redmine)