Support for executing queries in parallel

[andresmgot]

What would you like to be added:

Grafana's query service executes queries in parallel if they belong to different datasources but for each datasource, queries are always executed secuentially.

Some plugins implements their own logic when they want to execute queries in parallel. Some examples:

• Core sqleng: https://github.com/grafana/grafana/blob/main/pkg/tsdb/mssql/sqleng/sql_engine.go#L160
• sqlds (library used for multiple SQL datasources): https://github.com/grafana/sqlds/blob/71090eb4d9b6af7c745e2c238131e8d107e63191/datasource.go#L127-L148
• axiom datasource: https://github.com/axiomhq/axiom-grafana/blob/main/pkg/plugin/datasource.go#L130-L168

The goal of this issue is to provide a generic and easy-to-use solution for plugins that want to opt-in to execute queries in parallel.

Why is this needed: