Update TON Center API schema to include new decoder feature

[laviniat1996]

Toncenter API now decodes message bodies using hundreds of known TLB schemes and returns the result in the Message.message_content.decoded field. https://t.me/toncenter_news/55

We need to update the API refs to reflect this change.

[anton-trunov]

@laviniat1996 please add the corresponding parent issue