scheduler: merge Reserve and Unreserve plugins

Previously, separate interfaces were defined for Reserve and Unreserve
plugins. However, in nearly all cases, a plugin that allocates a
resource using Reserve will likely want to register itself for Unreserve
as well in order to free the allocated resource at the end of a failed
scheduling/binding cycle. Having separate plugins for Reserve and
Unreserve also adds unnecessary config toil. To that end, this patch
aims to merge the two plugins into a single interface called a
ReservePlugin that requires implementing both the Reserve and Unreserve
methods.

pkg/scheduler/framework/plugins/legacy_registry_test.go

@@ -110,12 +110,11 @@ func TestRegisterConfigProducers(t *testing.T) {
 				{Name: testScoreName2, Weight: 1},
 			},
 		},
-		Reserve:   &config.PluginSet{},
-		Permit:    &config.PluginSet{},
-		PreBind:   &config.PluginSet{},
-		Bind:      &config.PluginSet{},
-		PostBind:  &config.PluginSet{},
-		Unreserve: &config.PluginSet{},
+		Reserve:  &config.PluginSet{},
+		Permit:   &config.PluginSet{},
+		PreBind:  &config.PluginSet{},
+		Bind:     &config.PluginSet{},
+		PostBind: &config.PluginSet{},
 	}
 
 	if diff := cmp.Diff(wantPlugins, gotPlugins); diff != "" {