Add polls support

[MCausc78]

Description

Implement https://github.com/discord/discord-api-docs/pull/6746

I haven't tested sending message with poll (PollBuilder) and await message.endPoll(); since sending message with poll gives:

Unhandled exception:
403: Forbidden (0) POST https://discord.com/api/v10/channels/XXX/messages
#0      HttpHandler.executeSafe (package:nyxx/src/http/handler.dart:210:7)
<asynchronous suspension>
#1      MessageManager.create (package:nyxx/src/http/managers/message_manager.dart:412:22)
<asynchronous suspension>

endPoll:

Unhandled exception:
Cannot edit a message authored by another user (50005) POST https://discord.com/api/v10/channels/XXX/polls/YYY/expire
#0      HttpHandler.executeSafe (package:nyxx/src/http/handler.dart:210:7)
<asynchronous suspension>
#1      MessageManager.endPoll (package:nyxx/src/http/managers/message_manager.dart:682:22)
<asynchronous suspension>

After testing sending poll/endPoll, mark as ready for review

Connected issues & other potential problems

None

Type of change

Please delete options that are not relevant.

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• [x] Ran dart analyze or make analyze and fixed all issues
• [x] Ran dart format --set-exit-if-changed -l 160 ./lib or make format and fixed all issues
• [x] I have performed a self-review of my own code
• [x] I have commented my code, particularly in hard-to-understand areas
• [x] I have made corresponding changes to the documentation
• [ ] I have added tests that prove my fix is effective or that my feature works

[MCausc78]

Can you add tests?

Integration tests that send and end a poll would be nice.

Yeah, polls aren't locked under experiment now, i'll add them once i will at home

[abitofevrything]

Sounds great, thanks!