Add thread attribute to Message class

[Ratery]

Summary

Added thread attribute to the Message class. It returns the thread which was created from the message.
Also then message.create_thread is used message.thread will be automatically updated.

Checklist

• If code changes were made then they have been tested.
  • I have updated the documentation to reflect the changes.
• If type: ignore comments were used, a comment is also left explaining why
• 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, typehinting, examples, ...)

[plun1331]

I would make the updating of the thread match the various _handle_x methods

[Lulalaby]

Reminder:

⚠️ We currently have a feature freeze

[Ratery]

Reminder:

⚠️ We currently have a feature freeze

Yes, I know. But this PR might be merged before 2.1 version release.

[Dorukyum]

I think we can include this in 2.0 since threads have been added in 2.0 too.

[krittick]

Please resolve conflicts. Thanks!

[Dorukyum]

Resolved merge conflicts.

[plun1331]

Should this update a cached message's thread when a thread create/update event is received?

[Lulalaby]

Should this update a cached message's thread when a thread create/update event is received?

I would say yes