Allow for percentage of actors enablement per type

[jnunemaker]

Right now percentage of actors is shared across all types that are checked. This is easiest to understand with a use case. Let's say you want to check if something is enabled by user and by some other type (say project, organization, repository, etc.). Currently, if you set % of actors to 20, that means 20% of users and projects (or any other type) would all be enabled.

It would be neat to be able to users for 20% and not enable % of actors for other types or any other number of combinations. I have no idea how I want to do this yet, but I want to at least track it as an issue so if others hit this they have a place to voice for or against or even start hacking.

The big thing is we'd need adapter storage for options or some other sort of meta data per feature (or gate value). #163 would make storage easy if I finally pull the trigger on that.

We'll also need a standard way to deserialize flipper_id's into type/id. Currently people seem to use colon or semicolon, but likely we'll need an actual serializer/deserializer with a default of semicolon and then allow people to use a custom serializer if they need different.

[jnunemaker]

#557 (when finished) makes this possible so I'm closing this.

https://github.com/jnunemaker/flipper/blob/141be3c40eab072b19cc4dd5e8e795f155db40e4/examples/expressions.rb#L158-L174