[BUG] EnginePlugin always requires user to implement getEngineFactory

[jmazanec15]

Describe the bug
EnginePlugin interface requires plugins to implement getEngineFactory. However, when a user just wants to implement getCustomCodecServiceFactory, it requires them to implement this method and return Optional.empty().

To Reproduce
Implement EnginePlugin.

Expected behavior
getEngineFactory to default to empty.

I can submit a fix for this.

[jmazanec15]

For ref, here is example of problem.