Allow RPC to use secrets and buttons at the same time (by replacing discord default buttons with the provided custom ones)

Currently when you send an activity to RPC that has both secrets and buttons, discord will respond with a message: 'secrets cannot currently be sent with buttons'. This feels like unnecessary limitation.

Custom buttons should just replace discord's "Ask to Join" and "Spectate" buttons or allow the application to decide the desired behaviour between replacing the defaults or appending to them (or the application can decide to not send custom buttons if it wants the default ones)