Add support for jvm.options.d direcotry

[kresss]

It would be nice if logstash supported a jvm.options.d directory to put in new JVM Options. This is particularly useful for RPM installs where if you change the jvm.options file it will persist through an upgrade and prevent changes from Elastic from being applied. That scenario can cause logstash to fail due to incompatible jvm.options after a JVM upgrade that shipped with logstash.

[kresss]

See elastic/elasticsearch#51882

[fholzer]

Not having this feature can cause issues when upgrading logstash after modifying /etc/logstash/jvm.options.