feat: Add the shared client theme object on message sent and in message object

[UnBonWhisky]

Caution

Feature freeze is active, as described by our Release Schedule.

Summary

It adds the object of shared client theme described here by Discord.

Yes AI has been used (partially)

Information

• 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, ...).

Checklist

• I have searched the open pull requests for duplicates.
• 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.
• I have updated the changelog to include these changes.
• AI Usage has been disclosed.
  • If AI has been used, I understand fully what the code does

[pycord-app]

Thanks for opening this pull request!
Please make sure you have read the Contributing Guidelines and Code of Conduct.

This pull request can be checked-out with:

git fetch origin pull/3209/head:pr-3209
git checkout pr-3209

This pull request can be installed with:

pip install git+https://github.com/Pycord-Development/pycord@refs/pull/3209/head

[UnBonWhisky]

Note : shared client theme should not work with components v2 (source: Discord docs) but it works.