Add jars dynamically to Mleap Server

[akarsh3007]

Currently, If we write custom transformers we need to rebuild the mleap-serving docker container with the custom jars. It would be a great feature if we can add the jars dynamically or use sbt-native-packager and include mleap-serving as a dependency.

So essentially you could start the server with an option --mleap-jar https://example.com/my-assembly.jar

[akarsh3007]

@hollinwilkins can I take this issue?