Datasources: Add concurrency number to the settings

[itsmylife]

What is this feature?

Currently, only Loki (soon InfluxDB for influxql) is supporting concurrent query execution. Concurrency allows us to execute queries in parallel.

This ensures that the total execution time of multiple queries is only the longest query rather than the sum of all the query times.

Currently, data sources that use concurrency have a hard-coded concurrent query limit. This must be configurable.

Loki concurrency: #74064
InfluxDB concurrency: #81209

Why do we need this feature?

To be able to execute and return responses faster for the requests that have many queries.

Who is this feature for?

For the users that need to run multiple queries in one panel.
⚠️ Concurrency support is on the data source level. Concurrency must be supported by data sources.
Please refer data source documentation for more information

Special notes for your reviewer:
This setting option is not being used anywhere yet. After having this in place Loki and InfluxDB will start using this.

Please check that:

• It works as expected from a user's perspective.
• If this is a pre-GA feature, it is behind a feature toggle.
• The docs are updated, and if this is a notable improvement, it's added to our What's New doc.

[itsmylife]

Should we add this to the changelog?