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 0% with 42 lines in your changes missing coverage. Please review.
✅ Project coverage is 70.83%. Comparing base (8825b4a) to head (854322f).

Files with missing lines	Patch %	Lines
tests/dist.rs	0.00%	27 Missing ⚠️
src/bin/sccache-dist/main.rs	0.00%	9 Missing ⚠️
tests/harness/mod.rs	0.00%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2714      +/-   ##
==========================================
- Coverage   74.41%   70.83%   -3.58%     
==========================================
  Files          70       70              
  Lines       39211    39137      -74     
==========================================
- Hits        29177    27723    -1454     
- Misses      10034    11414    +1380     

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