feat(cmd): add kong-admin-token flag

[VarunAthreya]

Add --kong-admin-token flag for Kong Admin API authentication

Summary

Adds a new --kong-admin-token persistent flag that lets users authenticate against Kong's Admin API. The token is injected as a Kong-Admin-Token header on requests to the gateway.

GH: #1730

Changes

• New --kong-admin-token flag registered on the root command. It can also be set via the DECK_KONG_ADMIN_TOKEN environment variable (bound through Viper), consistent with deck's other config options.
• Refactored extendHeaders to accept a variadic list of optional headers via a new header{name, value} struct. Headers with an empty value are skipped, so the Kong-Admin-Token header is only added when a token is actually provided.
• Wired the token into initConfig, appending the Kong-Admin-Token header alongside the existing User-Agent header.

Notes

• Empty tokens are a no-op — no header is sent if the flag/env var is unset, preserving existing behavior.
• The variadic refactor keeps the existing User-Agent injection intact while making the helper reusable for future headers.

[CLAassistant]

All committers have signed the CLA.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 31 lines in your changes missing coverage. Please review.
✅ Project coverage is 36.48%. Comparing base (25fdf95) to head (33af6ff).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
cmd/root.go	0.00%	27 Missing ⚠️
tests/integration/test_utils.go	0.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2123      +/-   ##
==========================================
+ Coverage   33.17%   36.48%   +3.30%     
==========================================
  Files          78       78              
  Lines        7433     7477      +44     
==========================================
+ Hits         2466     2728     +262     
+ Misses       4763     4503     -260     
- Partials      204      246      +42     

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