Datanode config separation (#15701)

* Different variants of security configuration (from file, from mongo, insecure).
Encryption/decryption pattern used in practice.

* Reverting temporary changes

* More precise certificate presence check

* Added unit tests for testHasCert() method

* Datanode and opensearch configuration refactored

* code cleanup

* code cleanup

* code cleanup

* Node preflight config service refactored

* restbaseurl moved to OpensearchConfiguration

---------

Co-authored-by: Lukasz Kaminski <lukasz.kaminski@graylog.com>

data-node/src/test/java/org/graylog/datanode/periodicals/NodePingPeriodicalTest.java

@@ -36,7 +36,7 @@ class NodePingPeriodicalTest {
         final NodePingPeriodical task = new NodePingPeriodical(
                 nodeService,
                 nodeID,
-                uri,
+                () -> uri,
                 () -> true
         );
 
@@ -62,7 +62,7 @@ class NodePingPeriodicalTest {
         final NodePingPeriodical task = new NodePingPeriodical(
                 nodeService,
                 nodeID,
-                uri,
+                () -> uri,
                 () -> true
         );