Read only endpoint support (#10)

* This change introduces ability to set additional endpoint leading to the
instance of the graphql server connected to the read only database.
If regular query is detected and endpoint for `HOST_GRAPHQL_READONLY` value is set,
the query will be proxied to it. Mutations and non-graphql will be sent
to the `HOST_GRAPHQL` endpoint.

struct_config.go

@@ -33,16 +33,17 @@ type config struct {
 		BlockIntrospection   bool
 	}
 	Server struct {
-		HostGraphQL        string
-		HealthcheckGraphQL string
-		AllowURLs          []string
-		PortGraphQL        int
-		PortMonitoring     int
-		ApiPort            int
-		PurgeEvery         int
-		AccessLog          bool
-		ReadOnlyMode       bool
-		EnableApi          bool
-		PurgeOnCrawl       bool
+		HostGraphQL         string
+		HostGraphQLReadOnly string
+		HealthcheckGraphQL  string
+		AllowURLs           []string
+		PortGraphQL         int
+		PortMonitoring      int
+		ApiPort             int
+		PurgeEvery          int
+		AccessLog           bool
+		ReadOnlyMode        bool
+		EnableApi           bool
+		PurgeOnCrawl        bool
 	}
 }