Include whether the bot is a member in the self member guilds endpoint

[nikammerlaan]

Motivation

In the Get Current User Guilds endpoint, you can get a list of an oauth'd user's guilds. However, the response does not indicate which of the guilds currently have invited the bot. In our application, we'd optimally display a list of the user's guilds and clearly differentiate which ones are already setup and which haven't been.

Alternatives

For a bot that connects to the gateway, you could cross reference that information with the user's guilds to find which ones have the bot invited. However, not all bots have gateway connections -- especially the new breed of interaction-only bots. Additionally, this becomes much harder for bots at scale because the gateway information may be spread out. For our application, this was infeasible because we do not connect to the gateway.

One could also use the Get Guild endpoint, but the excessive number of requests needed requires that it's only retrieved lazily. This is the approach our application uses currently, but we were forced to compromise on the UX to achieve it.