[Proposal][Vega] Support PPL in vega visualization

[ruanyl]

Introduction

Currently, the OSD vega visualization only support OpenSearch query DSL. As PPL is also adopted and used by different features of OSD. I propose to also support PPL in vega visualization.

Proposal

(draft) I propose to extend the existing data.url to have a new %type%: ppl, the schema could be like:

{
  "data": {
        "url": {
            "%type%": "ppl",
            "query": "source=opensearch_dashboards_sample_data_logs| where QUERY_STRING(['response'], '4* OR 5*')"
        }
    }
}

Open question

1. PPL is a feature from a OpenSearch plugin, but vega is a core OSD plugin feature, having OSD core to "depend"(as it needs to call PPL endpoint to get data) on a OpenSearch plugin, will this be a concern even though the OpenSearch plugin will be present in main OpenSearch distributions?