Introduce partitioning modes

[ondrejbudai]

A very early PR for COMPOSER-1882 (internal only link, sorry!).

The new partitioning modes are:

• auto-lvm (the default, keeps the current behaviour)
• lvm (the image uses LVM even without extra partitions)
• raw (the image uses raw partitioning even with extra partitions)

Currently, they are only exposed in the cloud API.

To-do:

• discuss whether my changes in internal/disk are sane
• support in the Weldr API
• documentation for Weldr (in osbuild/guides)
• tests

This pull request includes:

• adequate testing for the new functionality or fixed issue
• adequate documentation informing people about the change such as
  • submit a PR for the guides repository if this PR changed any behavior described there: https://www.osbuild.org/guides/

[github-actions]

This PR is stale because it has been open 30 days with no activity. Remove "Stale" label or comment or this will be closed in 7 days.

[bcl]

Removed stale, I'm working on a variant of this and will keep this branch for reference.

[github-actions]

This PR is stale because it has been open 30 days with no activity. Remove "Stale" label or comment or this will be closed in 7 days.

[bcl]

#3723 and osbuild/images#189 are the current version of this.