Add independent environment reload

in the case that the prewarming pool is depleting (see PrewarmingPoolThreshold) and is still depleting after a timeout (PrewarmingPoolReloadTimeout).

internal/runner/aws_manager_test.go

@@ -112,5 +112,7 @@ func createBasicEnvironmentMock(id dto.EnvironmentID) *ExecutionEnvironmentMock
 	environment.On("NetworkAccess").Return(false, nil)
 	environment.On("DeleteRunner", mock.AnythingOfType("string")).Return(false)
 	environment.On("ApplyPrewarmingPoolSize").Return(nil)
+	environment.On("IdleRunnerCount").Return(uint(1)).Maybe()
+	environment.On("PrewarmingPoolSize").Return(uint(1)).Maybe()
 	return environment
 }