[storage] Add helper to storage extension for retrieving purger

[mahadzaryab1]

Which problem is this PR solving?

• Towards Remove usages of GetStorageFactory #6564

Description of the changes

• Adds a helper to get the purger from the jaeger storage extension.

How was this change tested?

• CI

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: npm run lint and npm run test

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.01%. Comparing base (ae4fb2d) to head (39797ba).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6694      +/-   ##
==========================================
+ Coverage   95.99%   96.01%   +0.01%     
==========================================
  Files         363      363              
  Lines       20558    20568      +10     
==========================================
+ Hits        19735    19748      +13     
+ Misses        628      626       -2     
+ Partials      195      194       -1     

Flag	Coverage Δ
badger_v1	9.82% <ø> (ø)
badger_v2	1.82% <0.00%> (-0.01%)	⬇️
cassandra-4.x-v1-manual	14.79% <ø> (ø)
cassandra-4.x-v2-auto	1.81% <0.00%> (-0.01%)	⬇️
cassandra-4.x-v2-manual	1.81% <0.00%> (-0.01%)	⬇️
cassandra-5.x-v1-manual	14.79% <ø> (ø)
cassandra-5.x-v2-auto	1.81% <0.00%> (-0.01%)	⬇️
cassandra-5.x-v2-manual	1.81% <0.00%> (-0.01%)	⬇️
elasticsearch-6.x-v1	19.12% <ø> (ø)
elasticsearch-7.x-v1	19.20% <ø> (ø)
elasticsearch-8.x-v1	19.37% <ø> (ø)
elasticsearch-8.x-v2	1.82% <0.00%> (-0.01%)	⬇️
grpc_v1	10.80% <ø> (ø)
grpc_v2	7.80% <0.00%> (-0.01%)	⬇️
kafka-3.x-v1	10.12% <ø> (ø)
kafka-3.x-v2	1.82% <0.00%> (-0.01%)	⬇️
memory_v2	1.82% <0.00%> (-0.01%)	⬇️
opensearch-1.x-v1	19.25% <ø> (ø)
opensearch-2.x-v1	19.25% <ø> (ø)
opensearch-2.x-v2	1.82% <0.00%> (-0.01%)	⬇️
tailsampling-processor	0.48% <0.00%> (-0.01%)	⬇️
unittests	94.89% <100.00%> (+0.01%)	⬆️

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