Implement Bot API 7.0 Reactions

[anfanik]

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

Reactions

• Added the classes ReactionTypeEmoji and ReactionTypeCustomEmoji representing different types of reaction.
• Added updates about a reaction change on a message with non-anonymous reactions, represented by the class MessageReactionUpdated and the field message_reaction in the class Update. The bot must explicitly allow the update to receive it.
• Added updates about reaction changes on a message with anonymous reactions, represented by the class MessageReactionCountUpdated and the field message_reaction_count in the class Update. The bot must explicitly allow the update to receive it.
• Added the method setMessageReaction that allows bots to react to messages.
• Added the field available_reactions to the class Chat.

Changes:

• Add SetMessageReaction request see setMessageReaction
• Add message_reaction and message_reaction_count updates to Update see Update
• Add available_reactions field to Chat see Chat
• Add ReactionTypeAdapter because of requirement to use discriminator field in ReactionType and use BotUtils#GSON in TelegramBotClient
• Move BotUtils to com.pengrad.telegrambot.utility package

[mircoianese]

Approve