Implementation of polls

[LeadcodeDev]

Polls introduced to Discord's HTTP API. See more here

Implementation

• [x] Implement object structures
• [x] Add polls to {channel}.send() method
• [x] Define logical constructor declaration
• [x] Create builder

Proposal

// Constructors
final poll = Poll(
  question: PollQuestion(content: 'What is your favorite programming language ?'),
  answers: [
    PollAnswer(content: 'Dart', emoji: PartialEmoji.fromEmoji(emoji!)),
    PollAnswer(content: 'Java', emoji: PartialEmoji.fromUnicode('🎓')),
    PollAnswer(content: 'Python'),
    PollAnswer(content: 'JavaScript'),
  ],
  expireAt: Duration(days: 6),
  isAllowMultiple: true,
  layout: PollLayout.initial);

// Builder
final poll = PollBuilder()
  .setQuestion(question: 'What is your favorite programming language ?')
  .addAnswer(answer: 'Dart', emoji: PartialEmoji.fromEmoji(emoji!))
  .addAnswer(answer: 'Java', emoji: PartialEmoji.fromUnicode('🎓'))
  .addAnswer(answer: 'Python')
  .addAnswer(answer: 'JavaScript')
  .setExpireAt(Duration(days: 6))
  .allowMultipleResponses(true)
  .setLayout(PollLayout.initial)
  .build();