feat: add once decorator to discord.Client

[OmLanke]

Summary

Adds a @client.once() decorator. Unlike @client.event or @client.listen(), functions decorated will run only once.
It wraps await client.wait_for(). Similar functionality is found in discord.js, as it extends Node's Event Emitter.

This is useful for having stuff run when the bot is ready for the first time, like database connections once ready and stuff.
(Side note - This can potentially be a race condition, as other events can start dispatching before db connects. Since there is no built in functionality to delay dispatching of ready until a function has returned, using the once decorator is the closest and simplest you can get)

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.

[codecov]

Codecov Report

Merging #1940 (8f535fc) into master (2e09718) will decrease coverage by 0.03%.
The diff coverage is 6.25%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1940      +/-   ##
==========================================
- Coverage   33.36%   33.33%   -0.03%     
==========================================
  Files          97       97              
  Lines       18851    18867      +16     
==========================================
+ Hits         6289     6290       +1     
- Misses      12562    12577      +15     

Flag	Coverage Δ
macos-latest-3.10	33.32% <6.25%> (-0.03%)	⬇️
macos-latest-3.11	33.32% <6.25%> (-0.03%)	⬇️
macos-latest-3.8	33.33% <6.25%> (-0.03%)	⬇️
macos-latest-3.9	33.33% <6.25%> (-0.03%)	⬇️
ubuntu-latest-3.10	33.32% <6.25%> (-0.03%)	⬇️
ubuntu-latest-3.11	33.32% <6.25%> (-0.03%)	⬇️
ubuntu-latest-3.8	33.33% <6.25%> (-0.03%)	⬇️
ubuntu-latest-3.9	33.33% <6.25%> (-0.03%)	⬇️
windows-latest-3.10	33.32% <6.25%> (-0.03%)	⬇️
windows-latest-3.11	33.32% <6.25%> (-0.03%)	⬇️
windows-latest-3.8	33.33% <6.25%> (-0.03%)	⬇️
windows-latest-3.9	33.33% <6.25%> (-0.03%)	⬇️

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Impacted Files	Coverage Δ
discord/client.py	27.05% <6.25%> (-0.68%)	⬇️

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