volume diffs do not appear in 'nomad job plan'

[tgross]

Discovered this while doing some preliminary work on #7877. Task group volume requests are missing a diff implementation (and associated tests), which means that they don't show up in plan outputs. I already have a failing unit test worked up, and I'll add these plus run thru the checklist again to make sure we're not missing any others.

In this example, I have a group with count=2 and have changed its volume request from read_only=true to read_only=false. The plan shows an update but not the reason why there's an update:

$ nomad job plan ./example.nomad
+/- Job: "example"
+/- Task Group: "cache" (2 in-place update)
  +/- Task: "redis" (forces in-place update)

Scheduler dry-run:
- All tasks successfully allocated.

Job Modify Index: 18
To submit the job with version verification run:

nomad job run -check-index 18 ./example.nomad

When running the job with the check-index flag, the job will only be run if the
job modify index given matches the server-side version. If the index has
changed, another user has modified the job and the plan's results are
potentially invalid.

And the job does update as expected:

vagrant@linux$ nomad job run ./example.nomad
==> Monitoring evaluation "59407ede"
    Evaluation triggered by job "example"
    Allocation "0c9fa865" modified: node "99b4b73b", group "cache"
    Allocation "86412123" modified: node "99b4b73b", group "cache"
==> Monitoring evaluation "59407ede"
    Evaluation within deployment: "34eb8867"
    Evaluation status changed: "pending" -> "complete"
==> Evaluation "59407ede" finished with status "complete"

[github-actions]

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