[exporter/debug] Add file output options for customLogger

[jackgopack4]

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Description

Adds ability to reroute output and error output to stdout/stderr/specified files in debugexporter when not using collector internal telemetry pipeline.

Link to tracking issue

Fixes #10472

Testing

Unit tests to ensure that output actually gets written to files when specified.

Documentation

Chloggen and update README.md for exporter/debug.

Notes

This would change the default behavior to outputting error output to stderr, I think that's reasonable given that it matches internal telemetry behavior, but since debugexporter is marked stable, We could just leave default for both to stdout or add a featuregate for this.