Introduce DiffResources struct

[criztovyl]

trafficstars

What this PR does / why we need it:

See #635

Which issue(s) this PR fixes:

Fixes #635

Does this PR introduce a user-facing change?

NONE

Additional Notes for your reviewer:

Review Checklist:

• [x] Follows the developer guidelines
• [x] Relevant tests are added or updated
• [x] Relevant docs in this repo added or updated
• [x] Relevant carvel.dev docs added or updated in a separate PR and there's a link to that PR
• [x] Code is at least as readable and maintainable as it was before this change

Additional documentation e.g., Proposal, usage docs, etc.:

[criztovyl]

This is a first take, I'll try to integrate this into my PR to demonstrate that it actually helps :)

[100mik]

Thanks for the PR! Will take a closer look at this.