Implement Bot API 7.0 Giveaways

[anfanik]

This pull request implements Giveaway part of Bot API 7.0 update:

Giveaway

• Added the class Giveaway and the field giveaway to the class Message for messages about scheduled giveaways.
• Added the class GiveawayCreated and the field giveaway_created to the class Message for service messages about the creation of a scheduled giveaway.
• Added the class GiveawayWinners and the field giveaway_winners to the class Message for messages about the completion of a giveaway with public winners.
• Added the class GiveawayCompleted and the field giveaway_completed to the class Message for service messages about the completion of a giveaway without public winners.

Changes: Add giveaway_created, giveaway, giveaway_winners and giveaway_completed to Message see Message

[anfanik]

Already implemented in https://github.com/pengrad/java-telegram-bot-api/pull/355