Refactor the upgrade plan command

**What would you like to be added**:

Figure out a better way to do the following checks [here](https://github.com/aws/eks-anywhere/pull/4503/files#diff-3f24cb161dffb0f13cccd0661ee7badcfcd326fc60da254a83ca383c2e2d85d8).

Discuss the use of pointers in ComponentsChangeDiff and if there is a different way of implementing it for upgrade vs upgrade plan command.

**Why is this needed**:

Start with the linked pull request diff, then trace the upgrade and upgrade plan command paths around ComponentsChangeDiff. Compare how the checks and pointer usage serve each command; done requires an agreed implementation direction for the refactor and confirmation that both command behaviors remain correct.

Written by the indexing model from the issue text.