Add functions that generalise stage option struct creation

[achilleas-k]

The functions in the stage_options.go file are the followup to the xStageOptions() methods from previous image type definitions (e.g., rpmStageOptions()) so they ended up in the same place for 8.5. They would be a lot more useful if they could be generalised and added to each stage in the osbuild2 package as analogues to the NewXStage() function. Right now they are only meant to be used in their respective distro package and there are several distro-specific hard-coded values.

Originally discussed in #1536 (comment)

[teg]

Would love to see things generalised.

The reason this was initially kept in each distro was to make sure that any policy was only defined in the individual distros, mainly to avoid a situation where we would change generic functions for the purposes of a new distro and accidentally changing them for old ones too (or just it being tricky to keep track of the consequences of changing things).

I think if the helpers are sufficiently generic, this shouldn't be a problem.