types 3.0: discriminated union types where appropriate

This issue comes out of #1227 and has one example of the suggestion to use discriminated unions in the draft PR #1228 (in this commit specifically).

Discriminated union types let us more accurately model certain kinds of either/or and hierarchical constraints. This article is one example goes into more detail about what this is and how to use it.

We also already use it extensively in the web-api package to model certain API request parameter constraints. The classic example where discriminated unions are useful within Slack is when modeling the parameters for the chat.postMessage method. This method requires one of text, blocks or attachments to be present - and with a discriminated union we model this constraint accurately!

Discriminated Union Usage Areas

• Message Attachments
• Block Kit - covered by #1905
• Event Payloads - covered by #1904

Start with the existing discriminated-union example in packages/web-api/src/types/request/chat.ts and review the linked chat.postMessage API documentation. Identify the remaining Message Attachments usage area and model its constraints with discriminated unions; the issue's checklist is complete when Message Attachments is covered, while Block Kit and Event Payloads are already covered by #1905 and #1904.