feat: set KIC's UserAgent

[pmalek]

What this PR does / why we need it:

This sets KIC's user agent so that it can be easily identified in Kubernetes Audit logs:

{
  "kind": "Event",
  "apiVersion": "audit.k8s.io/v1",
  "level": "Request",
  "auditID": "0f4fafa0-6f44-4c3a-b937-717a9cc6b042",
  "stage": "ResponseStarted",
  "requestURI": "/api/v1/services?allowWatchBookmarks=true&resourceVersion=362177&timeoutSeconds=367&watch=true",
  "verb": "watch",
  "user": {
    "username": "kubernetes-admin",
    "groups": [
      "system:masters",
      "system:authenticated"
    ]
  },
  "sourceIPs": [
    "172.18.0.1"
  ],
  "userAgent": "kong-ingress-controller/v2.12.0-189-g5a4d76638",
  "objectRef": {
    "resource": "services",
    "apiVersion": "v1"
  },
  "responseStatus": {
    "metadata": {},
    "code": 200
  },
  "requestReceivedTimestamp": "2023-10-30T17:16:50.566806Z",
  "stageTimestamp": "2023-10-30T17:16:50.567532Z",
  "annotations": {
    "authorization.k8s.io/decision": "allow",
    "authorization.k8s.io/reason": ""
  }
}

Without this change it uses the default user agent: "main/v0.0.0 (linux/arm64) kubernetes/$Format"

Special notes for reviewers

Is kubernetes-ingress-controller/<VERSION> something we want or do we want to use something different?

[programmer04]

🆒

in description of the PR it should be

"userAgent": "kong-ingress-controller/v2.12.0-189-g5a4d76638",

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (8c727d9) 75.3% compared to head (7eece07) 75.3%.
Report is 2 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #5019   +/-   ##
=====================================
  Coverage   75.3%   75.3%           
=====================================
  Files        167     167           
  Lines      18719   18721    +2     
=====================================
+ Hits       14097   14114   +17     
+ Misses      3794    3781   -13     
+ Partials     828     826    -2     

Files	Coverage Δ
internal/adminapi/backoff_strategy_konnect.go	100.0% <ø> (ø)
internal/adminapi/client.go	55.8% <ø> (ø)
internal/adminapi/endpoints.go	84.2% <ø> (ø)
internal/adminapi/kong.go	61.9% <ø> (ø)
internal/adminapi/konnect.go	0.0% <ø> (ø)
internal/admission/adminapi_provider.go	89.4% <ø> (ø)
internal/admission/handler.go	59.5% <ø> (ø)
internal/admission/utils.go	81.0% <ø> (ø)
...ion/validation/consumers/credentials/validation.go	96.2% <ø> (ø)
internal/admission/validation/gateway/httproute.go	90.0% <ø> (ø)
... and 109 more

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[pmalek]

🆒

in description of the PR it should be

"userAgent": "kong-ingress-controller/v2.12.0-189-g5a4d76638",

edited, thanks