feat: add /version endpoint and --version CLI flag

[mccutchen]

In order not to suddenly reveal potentially sensitive information without users explicitly opting in, the /version endpoint will only name the service:

{
  "service": "go-httpbin"
}

But with --use-full-version/USE_FULL_VERSION=true set, it exposes more detailed info:

{
  "service": "go-httpbin",
  "version": "2.22.0-next",
  "commit": "6c58644",
  "build_date": "2026-04-11T15:50:03Z",
  "go_version": "go1.26.1"
}

Closes #191, #190, and #188.

[codecov]

Codecov Report

❌ Patch coverage is 83.33333% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 94.72%. Comparing base (356a145) to head (9b31d96).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
httpbin/cmd/cmd.go	66.66%	3 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #248      +/-   ##
==========================================
+ Coverage   94.68%   94.72%   +0.04%     
==========================================
  Files          10        9       -1     
  Lines        1975     1991      +16     
==========================================
+ Hits         1870     1886      +16     
+ Misses         63       62       -1     
- Partials       42       43       +1     

Files with missing lines	Coverage Δ
httpbin/handlers.go	98.30% <100.00%> (-0.34%)	⬇️
httpbin/httpbin.go	100.00% <100.00%> (ø)
httpbin/options.go	93.61% <100.00%> (+1.30%)	⬆️
httpbin/cmd/cmd.go	88.78% <66.66%> (-0.51%)	⬇️

... and 1 file with indirect coverage changes

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[mloskot]

Thank you!

[mccutchen]

@mloskot thank you for the push to get this added! Just note that in the final implementation you’ll need to deploy it with USE_FULL_VERSION=1/-use-full-version to get detailed version info