Allow setting custom http headers for elasticsearch output

[AndrewChubatiuk]

Use Case

to ingest data from telegraf to VictoriaLogs it's required to remap telegraf fields to fields, which are supported by VictoriaLogs

Expected behavior

given below config, should allow properly ingest data into victorialogs from tail input

[[outputs.elasticsearch]]
  urls = ["http://localhost:9428/insert/elasticsearch"]
  timeout = "1m"
  flush_interval = "30s"
  enable_sniffer = false
  health_check_interval = "0s"
  index_name = "device_log-%Y.%m.%d"
  manage_template = false
  template_name = "telegraf"
  overwrite_template = false
  namepass = ["tail"]
  [outputs.elasticsearch.headers]
    "VL-Msg-Field" = "tail.value"
    "VL-Time-Field" = "@timestamp"
    "VL-Stream-Fields" = "measurement_name"

Actual behavior

Currently ingestion from telegraf doesn't create any records in VictoriaLogs

Additional info

No response