show preemptions in nomad plan CLI

[preetapan]

This PR augments the output of nomad plan with info about preemptions.

It uses a tiered approach to make sure the CLI doesn't show every single allocid if there are many preemptions. Instead, it groups things and shows summary counts.

There are three cases:

1 - Number of preempted allocations less than 10

Alloc ID  Job ID  Task Group
alloc1    jobID1  test
alloc2    jobID2  test2

2 - Number of unique jobids amongst all preemptions less than 10

Job ID  Namespace  Job Type  Preemptions
job0    test1       batch        3
job1    test2       batch        3
job2    test3       service      3
job3    test4       batch        3

3 - Number of unique jobids amongst all preemptions greater than 10

Job Type  Preemptions
batch     10
service   20

Builds on top of #4794, which should be merged first.

[schmichael]

[nit] Feel free to return early here and remove else. It would reduce nesting and make the big else clause easier to follow.

[schmichael]

goimport misformatting

[schmichael]

require2?

[github-actions]

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