Add architecture for multiple managers

using the chain of responsibility pattern.

internal/environment/abstract_manager.go

@@ -0,0 +1,40 @@
+package environment
+
+import (
+	"github.com/openHPI/poseidon/internal/runner"
+	"github.com/openHPI/poseidon/pkg/dto"
+)
+
+// AbstractManager is used to have a fallback environment manager in the chain of responsibility
+// following the null object pattern.
+type AbstractManager struct {
+	nextHandler ManagerHandler
+}
+
+func (n *AbstractManager) SetNextHandler(next ManagerHandler) {
+	n.nextHandler = next
+}
+
+func (n *AbstractManager) NextHandler() ManagerHandler {
+	return n.nextHandler
+}
+
+func (n *AbstractManager) List(_ bool) ([]runner.ExecutionEnvironment, error) {
+	return []runner.ExecutionEnvironment{}, nil
+}
+
+func (n *AbstractManager) Get(_ dto.EnvironmentID, _ bool) (runner.ExecutionEnvironment, error) {
+	return nil, runner.ErrNullObject
+}
+
+func (n *AbstractManager) CreateOrUpdate(_ dto.EnvironmentID, _ dto.ExecutionEnvironmentRequest) (bool, error) {
+	return false, nil
+}
+
+func (n *AbstractManager) Delete(_ dto.EnvironmentID) (bool, error) {
+	return false, nil
+}
+
+func (n *AbstractManager) Statistics() map[dto.EnvironmentID]*dto.StatisticalExecutionEnvironmentData {
+	return map[dto.EnvironmentID]*dto.StatisticalExecutionEnvironmentData{}
+}