move fiber selector logic from persistent plugin into its own sub-plugin

lib/httpx/plugins/persistent.rb

@@ -18,13 +18,18 @@ module HTTPX
     # https://gitlab.com/os85/httpx/wikis/Persistent
     #
     module Persistent
-      def self.load_dependencies(klass)
-        max_retries = if klass.default_options.respond_to?(:max_retries)
-          [klass.default_options.max_retries, 1].max
-        else
-          1
+      class << self
+        def load_dependencies(klass)
+          require_relative "fiber_selector"
+          klass.plugin(FiberSelector)
+
+          max_retries = if klass.default_options.respond_to?(:max_retries)
+            [klass.default_options.max_retries, 1].max
+          else
+            1
+          end
+          klass.plugin(:retries, max_retries: max_retries)
         end
-        klass.plugin(:retries, max_retries: max_retries)
       end
 
       def self.extra_options(options)
@@ -54,18 +59,6 @@ module HTTPX
             # consequences.
             (!ex.is_a?(RequestTimeoutError) || @options.max_retries != 1)
         end
-
-        def get_current_selector
-          super(&nil) || begin
-            return unless block_given?
-
-            default = yield
-
-            set_current_selector(default)
-
-            default
-          end
-        end
       end
     end
     register_plugin :persistent, Persistent

sig/plugins/persistent.rbs

@@ -9,6 +9,6 @@ module HTTPX
       end
     end
 
-    type sessionPersistent = Session & Persistent::InstanceMethods
+    type sessionPersistent = Session & FiberSelector::InstanceMethods & Persistent::InstanceMethods
   end
 end