DEFERRED_UPDATE_MESSAGE should display a loading state

[lipfangmoe]

It's in the title. The DEFERRED_UPDATE_MESSAGE acknowledgement should display a loading state, similar to what DEFERRED_CHANNEL_MESSAGE_WITH_SOURCE does.

Imagine you have an image gallery, with a string-select underneath. The problem is that the gallery needs to perform HTTP requests to third-party APIs in order to switch images, and those APIs tend to be a bit slow.

Since there's no guarantee on how fast the APIs are (often, they do end up exceeding 3 seconds), I send a DEFERRED_UPDATE_MESSAGE right away, which allows me to use the Edit Original Interaction Response to update the message that the string select is attached to.

However, I now have a new problem: I now have an unbounded wait, and when discord receives DEFERRED_UPDATE_MESSAGE, the loading state is removed. So the user of my application has no indication that work is still being done.

I guess a workaround would be to create an UPDATE_MESSAGE response to edit the previous message to add a "Loading..." text display... but it'd be a lot more convenient if DEFERRED_UPDATE_MESSAGE would just work the same way that DEFERRED_CHANNEL_MESSAGE_WITH_SOURCE does, leaving the loading indicator next to the string select until the Edit Original Interaction Response endpoint gets called.

I noticed that #3766 has a similar complaint. I decided to make a new topic here since that one is more about reworking how DEFERRED_UPDATE_MESSAGE should work rather than just simply asking that it doesn't erase the loading indicator