Gifts
- Added the class `UniqueGift` describing a gift that was upgraded to a unique one.
- Added the classes `UniqueGiftModel`, `UniqueGiftSymbol`, `UniqueGiftBackdropColors`,
  and `UniqueGiftBackdrop` to describe the properties of a unique gift.
- Added the class `UniqueGiftInfo` and the field `unique_gift` to the class `Message`,
  describing a service message about a unique gift that was sent or received.
- Added the class `GiftInfo` and the field `gift` to the class `Message`, describing a
  service message about a regular gift that was sent or received.
Also added new filters the new message types; `message.gift` and
`message.unique_gift`

Gifts
- Added the class `AcceptedGiftTypes` describing the types of gifts that
  are accepted by a user or a chat.

and tests.
Gifts
- Replaced the field `can_send_gift` with the field `accepted_gift_types` of
the type `AcceptedGiftTypes` in the class `ChatFullInfo`.

plus tests.
Business Accounts
- Added the classes `OwnedGiftRegular`, `OwnedGiftUnique`, `OwnedGifts` and
  the method `getBusinessAccountGifts`, allowing bots to fetch the list of gifts
  owned by a managed business account.

- Added the method `setBusinessAccountGiftSettings`, allowing bots to
  change the privacy settings pertaining to incoming gifts in a managed
  business account.
- Added the method `convertGiftToStars`, allowing bots to convert gifts
  received by a managed business account to Telegram Stars.
- Added the method `upgradeGift`, allowing bots to upgrade regular gifts
  received by a managed business account to unique gifts.
- Added the method `transferGift`, allowing bots to transfer unique gifts
  owned by a managed business account.

hooray.

Follow up to #4763. sorry forgot to push before merging.