Datasource: Make sure data proxy timeout applies to HTTP client (#25865)

For backend data sources executing in the backend (not through data proxy) make 
sure that the timeout applies to cached HTTP client.

Fixes #25863

Co-authored-by: Diana Payton <52059945+oddlittlebird@users.noreply.github.com>

conf/sample.ini

@@ -127,7 +127,8 @@
 # This enables data proxy logging, default is false
 ;logging = false
 
-# How long the data proxy should wait before timing out default is 30 (seconds)
+# How long the data proxy waits before timing out, default is 30 seconds.
+# This setting also applies to core backend HTTP data sources where query requests use an HTTP client with timeout set.
 ;timeout = 30
 
 # If enabled and user is not anonymous, data proxy will add X-Grafana-User header with username into the request, default is false.
@@ -233,7 +234,7 @@
 ;min_refresh_interval = 5s
 
 # Path to the default home dashboard. If this value is empty, then Grafana uses StaticRootPath + "dashboards/home.json"
-;default_home_dashboard_path = 
+;default_home_dashboard_path =
 
 #################################### Users ###############################
 [users]
@@ -459,7 +460,7 @@
 ;from_name = Grafana
 # EHLO identity in SMTP dialog (defaults to instance_name)
 ;ehlo_identity = dashboard.example.com
-# SMTP startTLS policy (defaults to 'OpportunisticStartTLS') 
+# SMTP startTLS policy (defaults to 'OpportunisticStartTLS')
 ;startTLS_policy = NoStartTLS
 
 [emails]