Add Trace support to SA Receiver

[rmfitzpatrick]

These changes adopt the converter introduced in #178 and add trace receiver support to the Smart Agent Receiver. They also port the extra and default span tags functionality from the SA codebase.

This is largely done to support the signalfx-forwarder monitor, whose documentation I will add in a subsequent PR once #167 has landed.

[codecov]

Codecov Report

Merging #192 (d78a1c1) into main (a1c52ac) will increase coverage by 0.30%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #192      +/-   ##
==========================================
+ Coverage   88.58%   88.88%   +0.30%     
==========================================
  Files          18       18              
  Lines         981     1008      +27     
==========================================
+ Hits          869      896      +27     
  Misses         78       78              
  Partials       34       34              

Impacted Files	Coverage Δ
internal/receiver/smartagentreceiver/factory.go	100.00% <100.00%> (ø)
internal/receiver/smartagentreceiver/output.go	93.16% <100.00%> (+1.00%)	⬆️
internal/receiver/smartagentreceiver/receiver.go	91.17% <100.00%> (+0.45%)	⬆️

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[pmcollins]

LGTM