fix: set SNI's certificate ID ref

[pmalek]

What this PR does / why we need it:

This PR adds SNI's certificate ID ref to be set in generated config. This basically means that instead of:

{
	"_format_version": "3.0",
	"certificates": [
		{
			"id": "8aade13c-1470-46bd-9849-9a74e349214f",
			"cert": "-----BEGIN CERTIFICATE-----\n...\n-----END CERTIFICATE-----",
			"key": "-----BEGIN RSA PRIVATE KEY-----\n...\n-----END RSA PRIVATE KEY-----",
			"snis": [
				{
					"name": "3.example.com"
				},
				{
					"name": "4.example.com"
				}
			]
		}
	]
}

we generate

{
	"_format_version": "3.0",
	"certificates": [
		{
			"id": "8aade13c-1470-46bd-9849-9a74e349214f",
			"cert": "-----BEGIN CERTIFICATE-----\n...\n-----END CERTIFICATE-----",
			"key": "-----BEGIN RSA PRIVATE KEY-----\n...\n-----END RSA PRIVATE KEY-----",
			"snis": [
				{
					"name": "3.example.com",
					"certificate": {
						"id": "8aade13c-1470-46bd-9849-9a74e349214f"
					}
				},
				{
					"name": "4.example.com"
					"certificate": {
						"id": "8aade13c-1470-46bd-9849-9a74e349214f"
					}
				}
			]
		}
	]
}

Which issue this PR fixes:

Fixes #6642

Special notes for your reviewer:

Related slack thread: https://kongstrong.slack.com/archives/C03CTMSHP6C/p1730991424228489

PR Readiness Checklist:

Complete these before marking the PR as ready to review:

• the CHANGELOG.md release notes have been updated to reflect any significant (and particularly user-facing) changes introduced by this PR

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 77.7%. Comparing base (565eb08) to head (618fd00).
Report is 10 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #6660   +/-   ##
=====================================
  Coverage   77.6%   77.7%           
=====================================
  Files        201     200    -1     
  Lines      23766   23681   -85     
=====================================
- Hits       18456   18402   -54     
+ Misses      4359    4340   -19     
+ Partials     951     939   -12     

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[pmalek]

Marking this as ready for review but this still requires feedback from Gateway team in the linked slack thread.

[czeslavo]

Very interesting find 👀 I wasn't aware KIC sets IDs for Certificates. Apparently it does so from the beginning using Kubernetes Secret's UID. Let's see what the Gateway team says about the Consumers triggering SNIs with no Certificate ID rejected. 🤔

Anyway, I think we can merge this as an improvement.