[FEATURE] Add support for promql in dashboards for querying prometheus datasources.

[vmmusings]

We currently have support for querying prometheus stores using promql via prometheus datasource connector.
Prometheus Connector: https://github.com/opensearch-project/sql/blob/main/docs/user/ppl/admin/connectors/prometheus_connector.rst.
PromQL Support: opensearch-project/sql#623

The requirement here is to add support for promql in dashboards for query prometheus datasource types. This feature might also require enhancements in the backend to support autocomplete and other features. We can create required backend issues when we have a design from the front POV.

<UX: TBA>

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