fix bug: get event value with key

[konoha279]

In function eventValue in evaluator/evaluate.go, if type event is interface{}, function can't return true value. Example:

- Event:
{
    "log": {
      "source": {
        "name": "aws",
        "product": "cloudtrail",
        "vendor": "aws"
      }
    }
}

- Key: "log.source.name"
- Reality return value of function: null
- True return value: "aws"

To fix this bug, need to flatten event to be able to get value by key string. Example:

- Event:
{
    "log": {
      "source": {
        "name": "aws",
        "product": "cloudtrail",
        "vendor": "aws"
      }
    }
}
- Flatten event:
{
  "log.source.name": "aws",
  "log.source.product": "cloudtrail",
  "log.source.vendor": "aws"
}

And of course we also need to unify the output of the eventValue function as an array by using toGenericSlice, to avoid the case where the return value of the eventValue function is an array. Example:

- Event:
{
  "log": [
    {
      "key": 1,
      "value": "a",
    },
    {
      "key": 2,
      "value": "b",
    }
  ],
  "temp": "abc"
}
- Flatten event:
{
  "log.key": [1, 2],
  "log.value": ["a", "b"]
}

With "[]interface{}{eventValue(event, field)}" :
- Key: "log.key"
- Value: error

- Key: "temp"
- Value: ["temp"]

With "toGenericSlice(eventValue(event, field))" :
- Key: "log.key"
- Value: [1,2]

- Key: "temp"
- Value: ["temp"]