Add support for multisubscriptions

[DA-344]

Summary

Adds support for multi-subscriptions.

DDevs PR: discord/discord-api-docs#7304

Checklist

• If code changes were made then they have been tested.
  • I have updated the documentation to reflect the changes.
• This PR fixes an issue.
• This PR adds something new (e.g. new method or parameters).
• This PR is a breaking change (e.g. methods or parameters removed/renamed)
• This PR is not a code change (e.g. documentation, README, ...)

[Rapptz]

This looks okay, it's a bit low level though so it doesn't feel particularly useful though the state of the library at this point when it comes to this is something I'm still fighting in my head. Anyway, doesn't impact this PR so it's good to go.