Add Get/Find/Exist extensions for Stats types that work without allocations

[AntyaDev]

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// gets scenario stats: 
// it throws exception if "scenario_1" is not found 
var scnStats1 = result.ScenarioStats.Get("scenario_1");

// finds scenario stats: 
// it returns null if "scenario_2" is not found 
var scnStats2 = result.ScenarioStats.Find("scenario_2");

// check that step "login" exist in the list
bool isLoginExist = scnStats.StepStats.Exists("login");

// check that status code "503" exist in the list
bool isExist = scnStats.Fail.StatusCodes.Exists("503");

// get stats of status code "503"
// in case of status not found, the exception will be thrown
var statusCode = scnStats.Fail.StatusCodes.Get("503");