torcontrol: Use fs::path instead of std::string for private key path
torcontrol: Use fs::path instead of std::string for private key path

Summary: Extract from core PR9902

Test Plan:
  make check

Reviewers: schancel, #bitcoin_abc

Reviewed By: schancel, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D893

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