Forward grafana meta values to downstream API

[yesoreyeram]

This PR will allow datasource authors to pass grafana meta data such as user id, datasource uid to the underlying API as query parameters / headers.

Any value set by the user in query will be overriden by the values from the data source settings.

Fixes #1073
Fixes #867
Fixes #457

How to test this

Before changes

• add a new infinity datasource instance
• in datasource custom http header add a new key X-Grafana-User with value ${__user.login}
• in datasource URL query parameter add a new key X-Grafana-Datasource-UID with value ${__ds.uid}
• Add https://httpbin.org to the allowed hosts
• Save and test the datasource
• Create a query in explore with URL and backend parser option
• Add https://httpbin.org/get as URL and run the query. Inspect headers and args in the result

After changes

refresh the query and you will see actual values / interpolated values in the args / headers as shown below.

also in query inspector