feat: set oom score for ssm-agent (#2618)

Add systemd drop-ins for this service that it won't get OOM-killed before
Guaranteed pods. See #2175.

Author: fletcherw

templates/al2023/runtime/rootfs/etc/systemd/system/amazon-ssm-agent.service.d/00-oom-score.conf

@@ -0,0 +1,4 @@
+[Slice]
+# Ensure that this is not OOM killed before Kubernetes Guaranteed QoS pods.
+# https://github.com/awslabs/amazon-eks-ami/issues/2175
+OOMScoreAdjust=-999