make gpud port number configurable

[aimichal]

I'd like to be able to choose the port number that gpud exposes.

This change renames DefaultGPUdPort to defaultGPUdPort to scope its visibility to the config package, and exposes a function GPUdPortNumber() that reads env var GPUD_PORT but falls back to defaultGPUdPort if it's missing or not an int.

It would be nice to factor this such that the port number is considered only when building the config (in DefaultConfig()) with a nice error message if it's malformed. But today DefaultGPUdPort (now GPUdPortNumber() is used throughout the codebase, so I decided to propose this surgical change to start.

[codecov]

Codecov Report

❌ Patch coverage is 47.05882% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 70.01%. Comparing base (fcc10e6) to head (cb24aca).
⚠️ Report is 52 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/config/default.go	55.55%	4 Missing ⚠️
cmd/gpud/status/command.go	0.00%	2 Missing ⚠️
cmd/gpud/list-plugins/command.go	0.00%	1 Missing ⚠️
cmd/gpud/run-plugin-group/command.go	0.00%	1 Missing ⚠️
cmd/gpud/set-healthy/command.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1199      +/-   ##
==========================================
- Coverage   70.01%   70.01%   -0.01%     
==========================================
  Files         339      339              
  Lines       24229    24237       +8     
==========================================
+ Hits        16965    16970       +5     
- Misses       6164     6166       +2     
- Partials     1100     1101       +1     

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[gyuho]

@aimichal The different port for listener is already supported by --listen-address flag, but some places you are changing are NOT using the flag value (as you correctly attempted to fix in this PR). Can we fix that instead, to use --listen-address flag value everywhere, rather than introducing the env variable?