[FEAT] Allow for changing bot banner for current bot

[AstreaTSS]

Problem Description

Bots recently gained support for bots to have banners. Needless to say, you should be able to set a bot's banner through interactions.py.

Proposed Solution

Add a banner attribute to ClientUser.edit and its respective functions. This should be relatively simple, since it's similar to avatar.

Alternatives Considered

No response

Additional Information

Discord docs technically don't reflect this, but it's in discord.py already, and has been publicly announced in the DDevs server.

Code of Conduct

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