fix(envoy-plugin): Source IP address resolution now uses x-forwarded-for header

[anurag-rajawat]

Description

Fixes # (issue)
This PR fixes the plugin to use the x-forwarded-for header to determine the real client (source) IP address. If x-forwarded-for is not present, the source address is used as a fallback.

Does this PR introduce a breaking change?
No.

Checklist

• PR title follows the <type>: <description> convention.
• I use conventional commits in my commit messages.
• I have updated the documentation accordingly.
• I Keep It Small and Simple: The smaller the PR is, the easier it is to review and have it merged.
• I have performed a self-review of my code.
• I have added tests that prove my fix is effective or that my feature works.
• New and existing unit tests pass locally with my changes.

Additional information for reviewer

Sample Event, some fields are removed for brevity:

{
 "metadata": {
  "context_id": 27,
  "timestamp": 1739469010,
  "receiver_name": "Istio",
  "receiver_version": "1.23.2"
 },
 "source": {
  "ip": "2401:4900:1c7a:133f:369c:76b3:ccb3:2e15",
  "port": 62213
 },
 "destination": {
  "ip": "10.244.0.43",
  "port": 8443
 },
 "request": {
  "headers": {
   "x-forwarded-for": "2401:4900:1c7a:133f:369c:76b3:ccb3:2e15,10.7.0.15",
   "x-forwarded-proto": "https",
   "x-request-id": "7ee7a9f8-87bf-406d-9b08-f2720ed3f585"
  }
 },
 "protocol": "HTTP/2"
}

Mention if this PR is part of any design or a continuation of previous PRs

[PrimalPimmy]

LGTM