Add custom time format to JSON and TEXT logging

[AlexFenlon]

Proposed changes

Add the option to change the timestamp of the logs in json and text format to unix in seconds, milliseconds and nanoseconds

eg. JSON with unix
{"time":"1755613721","level":"DEBUG","source":{"file":"controller.go","line":1019},"msg":"Syncing default/my-release-nginx-ingress-controller-7tj7f"}

default json
{"time":"2025-08-19T14:29:17.492029463Z","level":"DEBUG","source":{"file":"controller.go","line":1019},"msg":"Syncing default/my-release-nginx-ingress-controller-7tj7f"}

TEXT with unix-ms
time=1755619167305 level=DEBUG source=controller.go:1019 msg="Syncing default/my-release-nginx-ingress-controller-c828v"

default TEXT
time=2025-08-19T14:31:26.485Z level=DEBUG source=controller.go:1019 msg="Syncing default/my-release-nginx-ingress-controller-7tj7f"

Checklist

Before creating a PR, run through this checklist and mark each as complete.

• I have read the CONTRIBUTING doc
• I have added tests that prove my fix is effective or that my feature works
• I have checked that all unit tests pass after adding my changes
• I have updated necessary documentation
• I have rebased my branch onto main
• I will ensure my PR is targeting the main branch and pulling from my branch from my own fork

[codecov]

Codecov Report

❌ Patch coverage is 67.50000% with 13 lines in your changes missing coverage. Please review.
✅ Project coverage is 53.38%. Comparing base (54159f9) to head (773a3a0).
⚠️ Report is 8 commits behind head on main.

Files with missing lines	Patch %	Lines
cmd/nginx-ingress/flags.go	0.00%	12 Missing ⚠️
cmd/nginx-ingress/main.go	96.42%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8168      +/-   ##
==========================================
+ Coverage   53.08%   53.38%   +0.29%     
==========================================
  Files          90       90              
  Lines       21778    21950     +172     
==========================================
+ Hits        11561    11717     +156     
- Misses       9740     9756      +16     
  Partials      477      477              

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[javorszky]

tiny tiny bit, but otherwise g2g

[javorszky]

this isn't needed here, because

1. either we've already made sure that the time log format is one of the four allowed values, in which case if it's not the other three, then the switch's default arm is enough, or
2. we haven't, and if we pass in some weird value like iso8601, which should not be allowed, that will also be ignored and rfc3339 will be printed, as per the original value coming from slog

[vepatel]

imo we can extend --log-format to accept text-unix and json-unix instead of adding a very specific flag