Ability to specify image size on creation

[jgiardino]

Currently, Image Builder generates images that are of fixed size. This is almost never the size that people need, because they want to, for example, install additional software or run software that has larger storage requirements (such as Image Builder itself). In practice, this means people always have to manually resize the generated image to the size they require.

To make this easier, let’s allow specifying the size when creating the image.

Acceptance criteria:

1. Image size can be set via the compose API (new size key) and in the front end
   • ideas for UI are captured in this document
   • source html/css for the mockups shown are included in codesandbox
2. The image list in the front end shows the image size as well
3. Images continue fulfilling alignment requirements (Azure wants 1MB, for example: https://docs.microsoft.com/en-us/azure/virtual-machines/linux/create-upload-generic)

Nice to have:

1. Image size can be set from composer-cli (maybe with --size?) as well.
2. Research suitable defaults and min / max values for all output formats.

The issue for the API is osbuild/osbuild-composer#145