feat: Guest invites

[elenakrittik]

Summary

discord/discord-api-docs#6247

Official Discord client seem to check for whether a member is a guest by asserting not some_member.joined_at, so we'll do the same.

According to docs ~~written by myself~~ invite flags is an optional but not nullable flag. Though if some invite doesn't have flags, it is functionally equivalent to flags = 0, so the flags property is kept out of the Invite's docstring's table.

While not technically being a breaking change (for a reason only Danny and God know Member.joined_at is already optional despite official docs stating that it is always present), this PR actually introduces a real case when it can be optional. I'm not sure whether this should be mentioned in changelog as a breaking change, so decide yourself.

Checklist

• [x] If code changes were made, then they have been tested
  • [x] I have updated the documentation to reflect the changes
  • [x] I have formatted the code properly by running pdm lint
  • [x] I have type-checked the code by running pdm 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, ...)