Add adapter to opts, allowing to add custom configuration to the adapter

lib/ex_force.ex

@@ -90,6 +90,7 @@ defmodule ExForce do
 
   - `:headers`: set additional headers; default: `[{"user-agent", "#{@default_user_agent}"}]`
   - `:api_version`: use the given api_version; default: `"#{@default_api_version}"`
+  - `:adapter`: use the given adapter with custom opts; default: `nil`, which causes Tesla to use the default adapter or the one set in config.
   """
   def build_client(instance_url_or_map, opts \\ [headers: [{"user-agent", @default_user_agent}]])
 
@@ -108,7 +109,7 @@ defmodule ExForce do
       {Tesla.Middleware.Compression, format: "gzip"},
       {Tesla.Middleware.JSON, engine: Jason},
       {Tesla.Middleware.Headers, Keyword.get(opts, :headers, [])}
-    ])
+    ], Keyword.get(opts, :adapter))
   end
 
   @doc """