feat: show individual build servers capacity in --dist-status

[iTrooz]

This PR adds a field servers to the status returned by the scheduler, and subsequently to the JSON returned by sccache --dist-status. The primary benefit of this (for me) is to have a in_progress status per-server, which can be useful for the user to have a view of his compilation cluster.

I plan to submit a PR to add Prometheus metrics to the scheduler/build servers later, which would include this information among other, so I'd understand if you closed this PR (although I still see avalue in being able to quickly tell which servers are being scheduled atm)

New JSON:

{
  "SchedulerStatus": [
    "https://sccache.example.com",
    {
      "num_servers": 5,
      "num_cpus": 60,
      "in_progress": 0,
      "servers": [
        {
          "address": "1.1.1.1:8080",
          "num_cpus": 12,
          "in_progress": 0
        },
        {
          "address": "2.1.1.1:8080",
          "num_cpus": 12,
          "in_progress": 0
        },
        {
          "address": "3.1.1.1:8080",
          "num_cpus": 12,
          "in_progress": 0
        },
        {
          "address": "4.1.1.1:8080",
          "num_cpus": 12,
          "in_progress": 0
        },
        {
          "address": "5.1.1.1:8080",
          "num_cpus": 12,
          "in_progress": 0
        }
      ]
    }
  ]
}

[codecov-commenter]

Codecov Report

❌ Patch coverage is 80.76923% with 10 lines in your changes missing coverage. Please review.
✅ Project coverage is 74.35%. Comparing base (8825b4a) to head (8e05101).
⚠️ Report is 19 commits behind head on main.

Files with missing lines	Patch %	Lines
src/bin/sccache-dist/main.rs	0.00%	10 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2714      +/-   ##
==========================================
- Coverage   74.41%   74.35%   -0.06%     
==========================================
  Files          70       70              
  Lines       39211    39424     +213     
==========================================
+ Hits        29177    29313     +136     
- Misses      10034    10111      +77     

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[codspeed-hq]

Merging this PR will improve performance by 4.55%

⚠️ Different runtime environments detected

Some benchmarks with significant performance changes were compared across different runtime environments,
which may affect the accuracy of the results.

Open the report in CodSpeed to investigate

⚡ 1 improved benchmark
✅ 59 untouched benchmarks
⏩ 4 skipped benchmarks¹

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
⚡	Simulation	normalize_win_path_typical	1.6 µs	1.5 µs	+4.55%

Tip

Curious why this is faster? Comment @codspeedbot explain why this is faster on this PR, or directly use the CodSpeed MCP with your agent.

---

_{Comparing iTrooz:dist_status_individual (8e05101) with main (8825b4a)}

Footnotes

1. 4 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩