[refactor] move root span handler into aggregator

[Pushkarm029]

Which problem is this PR solving?

• [jaeger-v2] Add remotesampling extension #5389 (comment)

Description of the changes

• Refactored handleRootSpan logic into a helper method in aggregator.go.

How was this change tested?

• not tested

Checklist

• I have read https://github.com/jaegertracing/jaeger/blob/master/CONTRIBUTING_GUIDELINES.md
• I have signed all commits
• I have added unit tests for the new functionality
• I have run lint and test steps successfully
  • for jaeger: make lint test
  • for jaeger-ui: yarn lint and yarn test

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.53%. Comparing base (901165c) to head (143fc0c).
Report is 6 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5478      +/-   ##
==========================================
+ Coverage   95.52%   95.53%   +0.01%     
==========================================
  Files         331      330       -1     
  Lines       16155    16156       +1     
==========================================
+ Hits        15432    15435       +3     
+ Misses        548      547       -1     
+ Partials      175      174       -1     

Flag	Coverage Δ
badger_v1	8.05% <ø> (ø)
badger_v2	1.93% <ø> (ø)
cassandra-3.x-v1	16.43% <ø> (ø)
cassandra-3.x-v2	1.85% <ø> (ø)
cassandra-4.x-v1	16.43% <ø> (ø)
cassandra-4.x-v2	1.85% <ø> (ø)
elasticsearch-7.x	?
elasticsearch-8.x	?
grpc_v1	9.51% <ø> (-0.02%)	⬇️
grpc_v2	7.58% <ø> (ø)
kafka	9.78% <ø> (ø)
opensearch-1.x	1.78% <ø> (+0.01%)	⬆️
opensearch-2.x	1.78% <ø> (ø)
unittests	93.93% <100.00%> (+0.01%)	⬆️

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