Adapt pipeline specific APIs to support multi pipeline

Currently, some of the apis assume only 1 pipeline exists. With the upcoming multiple pipeline feature, this needs to be addressed, with the caveat of keeping backwards compatibility.

Single Pipeline Assumptions:

## 1. Pipeline Info API

`GET /_node/pipeline`

```json
{
"host": "Joaos-MBP-5.lan",
"version": "6.0.0-alpha1",
"http_address": "127.0.0.1:9600",
"id": "3d3147fe-eb86-45e9-9d13-2fb83a7c1550",
"name": "Joaos-MBP-5.lan",
"pipeline": {
"workers": 4,
"batch_size": 125,
"batch_delay": 5,
"config_reload_automatic": false,
"config_reload_interval": 3
}
}
```

## 2. Pipeline Stats API

`GET /_node/stats/pipeline`

```json
{
"pipeline": {
"events": {
"duration_in_millis": 7863504,
"in": 100,
"filtered": 100,
"out": 100
},
"plugins": {
"inputs": [],
"filters": [
{
"id": "grok_20e5cb7f7c9e712ef9750edf94aefb465e3e361b-2",
"events": {
"duration_in_millis": 48,
"in": 100,
"out": 100
},
"matches": 100,
"patterns_per_field": {
"message": 1
},
"name": "grok"
},
{
"id": "geoip_20e5cb7f7c9e712ef9750edf94aefb465e3e361b-3",
"events": {
"duration_in_millis": 141,
"in": 100,
"out": 100
},
"name": "geoip"
}
],
"outputs": [
{
"id": "20e5cb7f7c9e712ef9750edf94aefb465e3e361b-4",
"events": {
"in": 100,
"out": 100
},
"name": "elasticsearch"
}
]
},
"reloads": {
"last_error": null,
"successes": 0,
"last_success_timestamp": null,
"last_failure_timestamp": null,
"failures": 0
}
}
```

## 3. Node Stats API

The node stats includes data from the pipeline stats api:

`GET /_node/stats/pipeline`

```json
{
"host": "Joaos-MBP-5.lan",
"version": "6.0.0-alpha1",
"http_address": "127.0.0.1:9600",
"id": "3d3147fe-eb86-45e9-9d13-2fb83a7c1550",
"name": "Joaos-MBP-5.lan",
"jvm": {
# ...
"uptime_in_millis": 108897
},
"process": {
#...
},
"pipeline": {
"events": {
"duration_in_millis": 0,
"in": 0,
"filtered": 0,
"out": 0
},
"plugins": {
"inputs": [],
"filters": [],
"outputs": [
{
"id": "92ddb0615336293c1757cac81c3bebfa19985e68-2",
"name": "stdout"
}
]
},
"reloads": {
"last_error": null,
"successes": 0,
"last_success_timestamp": null,
"last_failure_timestamp": null,
"failures": 0
},
"queue": {
"type": "memory"
}
},
"reloads": {
"successes": 0,
"failures": 0
},
"os": {}
}
```