Support the GET /oauth2/@me endpoint

[damccull]

Currently when using twilight_http's client.current_user_application() method you must use an existing bot token. For applications that don't have a bot token because they are pure webhook-interaction applications, this information must be manually supplied instead of being looked up.

This seems to be due to the existing API supporting only the endpoint GET /oauth2/applications/@me. Please also support the endpoint GET /oauth2/@me so that bearer token authentication can get similar information via a lookup.

See https://discord.com/channels/745809834183753828/958900681085976656/959103887799902231 for the conversation about this.