Add support for DALLE AI image generation

[icyavocado]

Proposal

Integrate DALL-E to command /dalle prompt

1. Add DALL·E API request, and
2. Trigger the bot using /dalle 'prompt' command (example: dalle a dog with sun glasses)
3. Various settings should be available:
   • Multiple images support, preview before posting support (same layout as /giphy UI)
   • Togglable command /dalle
   • Able to pass n (how many images) and size (the size of the image) to the prompt

Expected outcome

An option to enable/disable the command /dalle.

[icyavocado]

Add command /dalle to the app: a1db263

[icyavocado]

Using ISlashCommandPreviewItem and SlashCommandPreviewItemType to preview dalle image before sending out.
Similar to how giphy app is doing their preview section.

import { ISlashCommandPreviewItem, SlashCommandPreviewItemType } from '@rocket.chat/apps-engine/definition/slashcommands';