added ability to set event driven thread count

[mh013370]

Q	A
Bug fix?	no
New feature?	yes
API breaks?	no
Deprecations?	no
Related tickets	fixes #183
License	Apache 2.0

What's in this PR?

Adds ability to set the maximumEventDrivenThreadCount like you can currently set the maximumTimerDrivenThreadCount

Why?

Currently, you can only configure it via the UI which isn't ideal when you have flows deployed via nifikop & registry.

Additional context

Checklist

• [x] Implementation tested
• [x] Error handling code meets the guideline
• [x] Logging code meets the guideline
• [x] User guide and development docs updated (if needed)
• [x] Append changelog with changes

[mh013370]

I've fixed the build on this PR as i mimicked changes seen in fe69e9eddd7e10ed7cc8ab6aa8d9b1c1ea73d133