Add targets to create attestation input documents

[thresheek]

Proposed changes

This PR adds a few targets which will be used to generate inputs for attestation documents for pkg-oss builds.
Those targets are a dependency of main ones - base, module-% so packaging scripts will not need to call any new targets to generate them.

Files that are generated (attest-base or attest-module-$modulename) can be fetched from the build source dir, and concatenated accordingly to produce a full input document for a given build.

Currently implemented for debian/ to test the idea and gather comments, and if we settle on the implementation, it's trivial to apply the same changes for other distributions, and downstream forks like NGINX Plus packaging.

[oxpa]

I was thinking more about changing every module makefile to get an explicit list of dependencies. And then use the said list everywhere.
Checkout attest branch in the repo with my approach implemented for lua. The result is the same though and the change is smeared across the whole repository. So I'm not sure it's any good.

[thresheek]

Thanks for the suggestion, indeed it's best not to guess but have a rather rigid dependency as defined per Makefiles.

I went ahead and implemented the following:

• dependencies in modules for debian, alpine, rpm
• fixed variables naming in contrib to properly match the contrib names
• moved attestation targets to contrib/attestation.mak to be included in OS-specific Makefiles so we don't have to duplicate the code

For now, the attestation code generates space-separated output files. I originally wanted it to be tab-separated, but am having a hard time convincing gnu make to output those. Maybe we'll just stick to space-separated then.

[thresheek]

FWIW, decided to stick with the whitespaces (discussed with the security team)

[oxpa]

Красивое __

[oxpa]

even though the PR touches almost every file in the repository - it feels like renaming some variables under contrib and defining dependencies is a welcomed change. I don't remember the exact reason but I was curious why it wasn't done initially. We will probably find places to simplify stuff later on. Well done, great job :)