Support configure http proxy per datasource

Please include this information: ### What Grafana version are you using? Version 5.4.2 (commit: d812109, branch: HEAD) ### What datasource are you using? * Grafana-Metric-Cloud * Prometheus (locally) * Alert manager ### What OS are you running grafana on? RHEL-7 ### What did you do? I wanted to connect my internal Grafana instance to metrics which are stored in Grafana-cloud services. however, since our servers are blocked from the internet, i need to use proxy. Grafana does not allow setting a proxy per data-source (or at all), the workaround for proxy is to override `http_proxy` and `no_proxy`. this is not easily manageable in enterprise, where servers can have additional software which must not use the proxy. in my case, even the internal Grafana has multiple other datasources (on internal network) which should't go via the proxy. ### What was the expected result? I expect Grafana to allow setting http-proxy for datasource (either in the GUI or in provisioning), notice that setting it globally on machine is not good enough as one have to manage HTTP_PROXY and NO_PROXY manually, outside of the application. there was a similar issue for alerting here - #11962