Added agent's custom fields to table result

[jpros]

This action is to enable search and summary in the dashboard view.

References to web:
amidaware/tacticalrmm-web#7

[codecov]

Codecov Report

Merging #1468 (4c78922) into develop (b5eed69) will increase coverage by 0.00%.
The diff coverage is 100.00%.

❗ Current head 4c78922 differs from pull request most recent head 56124d2. Consider uploading reports for the commit 56124d2 to get more accurate results

@@           Coverage Diff            @@
##           develop    #1468   +/-   ##
========================================
  Coverage    81.13%   81.13%           
========================================
  Files          116      116           
  Lines         7791     7792    +1     
========================================
+ Hits          6321     6322    +1     
  Misses        1470     1470           

Impacted Files	Coverage Δ
api/tacticalrmm/agents/serializers.py	96.07% <100.00%> (+0.03%)	⬆️
api/tacticalrmm/tacticalrmm/settings.py	92.85% <100.00%> (ø)

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[wh1te909]

thank you. I just need to test it on larger installs as this will add an extra db hit and so we might need to add a prefetch to the view as well to not cause n+1 query problem

[wh1te909]

I added the prefetch to the view (ignore the failing tests, that's not related lol)