Use agent bundle-relative Collectd ConfigDir default

[rmfitzpatrick]

Instead of using the Smart Agent default for the collectd config directory setting value, these changes introduce an agent-bundle relative location (~${bundleDir}/run/collectd) for more intuitive organization.

Thanks to @bjsignalfx for initial leg work in #241

[codecov]

Codecov Report

Merging #263 (2ec35a5) into main (75dfd86) will increase coverage by 0.07%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #263      +/-   ##
==========================================
+ Coverage   86.83%   86.91%   +0.07%     
==========================================
  Files          20       20              
  Lines        1481     1490       +9     
==========================================
+ Hits         1286     1295       +9     
  Misses        160      160              
  Partials       35       35              

Impacted Files	Coverage Δ
internal/extension/smartagentextension/config.go	92.40% <100.00%> (+0.97%)	⬆️

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