Add struct to allow universal method of setting jvm parameters

A customer wanted to override the jvm heap settings for the Druid operator today and was unable to do this.
In this specific instance it was resolved by the addition of resource configs to the Druid operator, which barely missed the last release window and hence is only available in the nightly version at the moment.

In the future it may become necessary though to allow users to override jvm parameter in some operators and ideally we'll want to do this in a universal way.

We should publish a struct in the operator framework that can be used by operators to allow setting these settings.

We have considered multiple versions of this:

1. A HashMap<String, String>
2. A Vec
3. A custom struct with both a HashMap and a List to accomodate both

All of these have drawbacks and benefits, especiallly when considering how to merge these.
At the moment we tend towards option 2 as this is the most simple one.

The downside is, that is doesn't allow merging with higher level structs to for example override the value of -Xmaxmx=100 with -Xmaxmx=200 for a rolegroup, as this is only a string to us and we won't understand that it refers to the same setting.

We do think that this is an acceptable compromise, since hopefully the need to override values here should not be too frequent.

No file, test, or entry point is named in the issue. Start by locating the operator framework's public configuration types, then compare the HashMap, Vec, and custom-struct options against the stated merging requirements. Done means publishing a reusable JVM-parameter structure with an agreed representation and merge behavior.