Support unknown values

There are currently a number of enum/enum-like values in our API that resemble the finite amount of values a certain field can have (like activity types). However, since the nature of Discord's api is volatile and changes are often poorly communicated to the public, we can end up in a situation where we receive valid, undocumented values outside of the ones our library supports.

When we do, we end up throwing an Exception, which isn't the greatest experience for anyone involved. Instead, we could add an extra value to all the enums named Unknown and delegate all values outside of the acceptable range to that value. Preventing Exceptions from being raised and allowing us to process the information related to those values in some capacity.