feat: add server variable extensions

[geffersonFerraz]

Allow to use server variable extension:

	envVar := &v3.ServerVariable{
		Enum:       []string{"api.mycompany.com", "api-preprod.mycompany.com"},
		Extensions: orderedmap.New[string, *yaml.Node](),
	}
	envVar.Description = "Environment to use"
	envVar.Default = "api.mycompany.com"
	envTransforms := []map[string]interface{}{
		{
			"type": "translate",
			"translations": []map[string]string{
				{"from": "prod", "to": "api.mycompany.com"},
				{"from": "pre-prod", "to": "api-preprod.mycompany.com"},
			},
		},
	}
	envVar.Extensions.Set("x-transforms", createExtension(envTransforms))
	server.Variables.Set("env", envVar)

	mergedSpecA.Model.Servers = append(mergedSpecA.Model.Servers, server)

[daveshanley]

LGTM. Thank you for your contribution.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.64%. Comparing base (49a2921) to head (d26b0d2).
Report is 22 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #326   +/-   ##
=======================================
  Coverage   99.63%   99.64%           
=======================================
  Files         164      164           
  Lines       16586    16674   +88     
=======================================
+ Hits        16526    16614   +88     
  Misses         55       55           
  Partials        5        5           

Flag	Coverage Δ
unittests	99.64% <100.00%> (+<0.01%)	⬆️

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[daveshanley]

Two lines to cover and it's good to go.

[daveshanley]

Fix the tests and I can add this in!