Add config option to set Hikari keepalive property

bukkit/src/main/resources/config.yml

@@ -128,6 +128,12 @@ data:
     #   connection time limit.
     maximum-lifetime: 1800000 # 30 minutes
 
+    # This setting controls how frequently the pool will 'ping' a connection in order to prevent it
+    # from being timed out by the database or network infrastructure, measured in milliseconds.
+    # - The value should be less than maximum-lifetime and greater than 30000 (30 seconds).
+    # - Setting the value to zero will disable the keepalive functionality.
+    keepalive-time: 0
+
     # This setting controls the maximum number of milliseconds that the plugin will wait for a
     # connection from the pool, before timing out.
     connection-timeout: 5000 # 5 seconds