Allow passing grafana variables as url params.

[hsuanyi]

Our use case is to have a predefined , parameterized, SQL script on clickhouse server side; and the only info we need from grafana is those variables. For some context, clickhouse has this predefined script capability [1]

As a simple example. on sever side we have

<query>
                SELECT * FROM table
                WHERE name IN ({param1:String}
</query>

and on grafana-clickhouse plugin side, we take grafana variables and plug in to data source url as params:

'http://clickhouse_url:8123/query_param_with_url?param1=XYZ'

[1] https://clickhouse.com/docs/en/interfaces/http#predefined_query_handler