Slash Commands: dynamic choices

Edit: this has been implement with the autocomplete option

Description

Let bots dynamically generate choices when entering a slash command. This could be useful for something like a subscription bot where

/unsubscribe <subscription name>

could easily have more than [25](https://discord.com/developers/docs/interactions/slash-commands#a-quick-note-on-limits) choices. Manually entering the name could lead to confusion (spelling mistakes etc.). If the choices could be requested from the bot directly, only the subscriptions that the user has would be displayed.

Why this is needed

1. since a maximum of 25 choices is definitely not enough for some applications, the only option for bot developers is to use no choices and validate the chosen value when the interaction is received

2. even if there were no limit to choices, it would be easier for users to see only the relevant choices

Alternatives

Use not choices at all and validate the chosen option later.
Also, I could imagine this in some kind of bot UI ( like the [pollinator example](https://www.figma.com/proto/DDsdQLvBqmmPgZNgeNKmvf/Bot-Proposal?node-id=129%3A77&viewport=28579%2C-4007%2C6.748552322387695&scaling=min-zoom) )

Concerns

First of all, this would certainly mean more requests to discord servers and bot endpoints. Also, I have no suggestion how the client should handle/receive lots of possible choices (a limit would probably still be needed)