Break up the `SwitchPortSettingsManager` RPW into smaller independently testable chunks.

When working on #8213 I struggled a bit on how to write a test that a) captures #8207 in the current codebase, and then works with the fix in place. This is because the RPW implementation below has command and control intertwined with the diff calculations that feed into that c&c. If we could separate out the diff calculation part (similar to what I believe we're doing for blueprints) from the code carrying out actions based on those diffs, then we could test this code much more easily I think.

https://github.com/oxidecomputer/omicron/blob/37a2122e4be0d947c141833a695bf11a1c32f537/nexus/src/app/background/tasks/sync_switch_configuration.rs#L291-L1330