feat(activity): add undocumented fields

[shiftinv]

Summary

Adds the activity fields id and platform, in addition to the already existing sync_id and session_id, all of which are not part of the api docs (see https://github.com/discord/discord-api-docs/pull/2219/commits/5822b0bc97fa8ad1f9e87d0426a309cbf680c5d3). This PR adds the missing fields without documentation (hence the skip news label), such that they're still available as they're useful regardless; no guarantees are made regarding their types or presence/absence.

Depends on #685.

Checklist

• [x] If code changes were made, then they have been tested
  • [ ] I have updated the documentation to reflect the changes
  • [x] I have formatted the code properly by running task lint
  • [x] I have type-checked the code by running task pyright
• [ ] This PR fixes an issue
• [x] 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, ...)

[searchify7]

What can these fields do?

[searchify7]

Oh well fixed typo lol

[onerandomusername]

@shiftinv could you rebase this pr on master?

[shiftinv]

@shiftinv could you rebase this pr on master?

Done.