Datasource: Shared HTTP client provider for core backend data sources and any data source using the data source proxy (#33439)

Uses new httpclient package from grafana-plugin-sdk-go introduced 
via grafana/grafana-plugin-sdk-go#328. 
Replaces the GetHTTPClient, GetTransport, GetTLSConfig methods defined 
on DataSource model.
Longer-term the goal is to migrate core HTTP backend data sources to use the 
SDK contracts and using httpclient.Provider for creating HTTP clients and such.

Co-authored-by: Arve Knudsen <arve.knudsen@gmail.com>

devenv/docker/blocks/slow_proxy/docker-compose.yaml

@@ -4,4 +4,5 @@
     ports:
       - "3011:3011"
     environment:
-      ORIGIN_SERVER: "http://localhost:9090/"
+      ORIGIN_SERVER: ${ORIGIN_SERVER}
+      SLEEP_DURATION: ${SLEEP_DURATION}