fix(multi): Support grouping by resource in multi resource mode

[doronkg]

What this PR does / why we need it?

This PR adds support to use --group-by=resource flag in multi resource mode.
In addition, it improves the output by adding newlines in similar to --group-by=namespace output.

$ .\main.exe sa,clusterrole --group-by=resource -o json
{
  "ClusterRole": {
    "": [
      "aaa"
    ]
  },
  "ServiceAccount": {
    "kube-system": [
      "tes2t"
  }
}

PR Checklist

• This PR adds K8s exceptions (false positives)
• This PR adds new code
• This PR includes tests for new/existing code
• This PR adds docs

GitHub Issue

Closes #289

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 30 lines in your changes missing coverage. Please review.

Project coverage is 41.74%. Comparing base (bc2cca8) to head (7ef8742).

Files	Patch %	Lines
pkg/kor/multi.go	0.00%	29 Missing ⚠️
pkg/kor/formatter.go	0.00%	1 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##             main     #320      +/-   ##
==========================================
- Coverage   41.77%   41.74%   -0.03%     
==========================================
  Files          61       61              
  Lines        3179     3181       +2     
==========================================
  Hits         1328     1328              
- Misses       1637     1639       +2     
  Partials      214      214              

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[yonahd]

LGTM