`output.pipeline` setting does not work in standalone mode

[khushijain21]

Steps to reproduce

1. Create a pipeline via Kibana's dev tools

PUT _ingest/pipeline/error_pipeline
{
  "description": "Add timestamp and remove unwanted fields",
  "processors": [
    {
      "set": {
        "field": "error_key",
        "value": "this is error value"
      }
    }
  ]
}

2. Start elastic-agent with following config

agent.grpc:
  port: 6799
id: c6225df3-f78a-475f-ba7f-af00d1986211
revision: 2
outputs:
  default:
    type: elasticsearch
    hosts:
      - http://localhost:9200/
    username: admin
    password: testing
    preset: custom
    pipeline: error_pipeline
    compression_level: 0

agent.monitoring:
  enabled: false

inputs:
- type: filestream
  id: my-filestream-id
  streams:
  - id: filestream-id
    enabled: true
    paths:
    - /var/log/*.log

You can see logs ingested but they do not contain the field error_key: this is error value

Preliminary investigation shows that getPipeline function always returns empty string https://github.com/elastic/beats/blob/main/libbeat/outputs/elasticsearch/client.go#L424 and that defaultSelector is nil.