Extend experimentation.rb definitions hash with Flipper gate type

A great idea from @pcalder came up during the Growth Engineering Weekly call:

Can we add another key/value pair to the Gitlab::Experimentation::EXPERIMENTS hash that tells the framework what Flipper gate to check for an experiment being enabled or not?

The main use case being to make use of the percentage of actors gate rather than the current requirement of using the percentage of time gate.