conduit: Default to plugin data directory when possible.

[winder]

Summary

Use the new plugin data directory as a default location for the local ledger and file writer.

Test Plan

New unit tests.

[codecov]

Codecov Report

Merging #1366 (206dc3e) into develop (9267008) will increase coverage by 0.23%.
The diff coverage is 81.81%.

@@             Coverage Diff             @@
##           develop    #1366      +/-   ##
===========================================
+ Coverage    63.07%   63.30%   +0.23%     
===========================================
  Files           77       77              
  Lines        10477    10483       +6     
===========================================
+ Hits          6608     6636      +28     
+ Misses        3308     3281      -27     
- Partials       561      566       +5     

Impacted Files	Coverage Δ
...it/plugins/processors/blockprocessor/initialize.go	64.81% <66.66%> (+0.92%)	⬆️
...ugins/processors/blockprocessor/block_processor.go	76.22% <80.00%> (+9.00%)	⬆️
...duit/plugins/exporters/filewriter/file_exporter.go	76.78% <100.00%> (+1.31%)	⬆️
idb/postgres/postgres.go	64.71% <0.00%> (-0.14%)	⬇️
fetcher/fetcher.go	63.98% <0.00%> (+1.27%)	⬆️

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