feat: k2pow service

[acud]

Work in progress. As discussed with NJ, this implementation will:

• allow users to run a k2pow compute worker as a separate binary
• the worker will have a very simple HTTP API
  • the API will have create, read, update operations for jobs
  • right now all is done in-memory for the worker, no persistence of state
• the post service needs a new flag that will start it in a special mode, in which jobs of k2pow computations will call a specified HTTP hook with the necessary params
  • how the result comes back to the post service is still TBD (either a hook on the service, or wait for response)

closes #343

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 81.11111% with 51 lines in your changes missing coverage. Please review.

Project coverage is 93.09%. Comparing base (d6e2f51) to head (937d596). Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
k2pow-service/src/job_manager.rs	78.68%	26 Missing :warning:
src/pow/service.rs	76.71%	17 Missing :warning:
src/pow/randomx.rs	46.66%	8 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #362 +/- ## ========================================== - Coverage 94.08% 93.09% -1.00% ========================================== Files 27 29 +2 Lines 3383 3633 +250 ========================================== + Hits 3183 3382 +199 - Misses 200 251 +51 ```

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