cloudapi: Add optional image size support

[bcl]

This adds a 'size' parameter to the image_request object. It can be used to specify the minimum image size in bytes.

This behaves in the same way as the size parameter of the weldr API.

For raw images the root partition is grown to fill the available space. For LVM images the PV uses the available space, but the LV does not, leaving space available for other LVs to be created after boot.

See COMPOSER-1883

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/

[ondrejbudai]

FWIW this looks good. :)

[bcl]

Closing in favor of #3597