Fix plugin serialization and export plugin object on config

[camdencheek]

Description of Changes

Adds missing JSON marshal/unmarshal methods for plugin.Config and exports the (plugin.Config).Plugin field so that its fields can be used by the universal agent.

Please check that the PR fulfills these requirements

• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)
• Add a changelog entry (for non-trivial bug fixes / features)
• CI passes

[djaglowski]

Log Files	Logs / Second	CPU Avg (%)	CPU Avg Δ (%)	Memory Avg (MB)	Memory Avg Δ (MB)
1	1000	1.7069203	-0.12073231	128.6937	+0.53515625
1	5000	6.4139256	+0.20692492	137.82503	+5.035568
1	10000	12.58662	+0.13810635	144.90935	+0.21565247
1	50000	59.82734	-0.8890953	176.87218	-0.7428589
1	100000	117.53902	-1.0721664	229.95892	+1.063446
10	100	2.3966227	+0.13793921	133.73424	+0.23342896
10	500	6.879529	-0.5689583	138.38658	+2.5968475
10	1000	14.000286	-1.3105373	144.33823	-1.4273987
10	5000	64.57045	-1.6729431	182.46484	-1.7365265
10	10000	125.48417	-1.5639877	222.93845	-0.99542236

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@bb2acd5). Click here to learn what that means.
The diff coverage is 28.57%.

@@            Coverage Diff            @@
##             master     #157   +/-   ##
=========================================
  Coverage          ?   69.41%           
=========================================
  Files             ?       85           
  Lines             ?     4554           
  Branches          ?        0           
=========================================
  Hits              ?     3161           
  Misses            ?     1059           
  Partials          ?      334           

Impacted Files	Coverage Δ
operator/buffer/buffer.go	80.95% <0.00%> (ø)
plugin/config.go	60.00% <27.27%> (ø)
plugin/plugin.go	98.11% <100.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update bb2acd5...9271fac. Read the comment docs.