getVariant/TargetingFilter might accept any context type

[Eskibear]

          We're actively debating this now- but I think context here should be unknown. https://github.com/microsoft/FeatureManagement-Dotnet/pull/484/files#r1717629098

Originally posted by @rossgrambo in #13 (comment)

[Eskibear]

Given the possibility that we calculate audience context id based on any other property rather than userid, I agree that the context can be unknown (or to limit it to an object?).
A good thing is, here all properties in ITargetingContext are optional, meaning it’s compatible with an arbitrary object type.