Buyback mechanisms could be easily extended

Currently, a buyback account can be created using an `account_create` operation with an `extension` like this:

"buyback_options": {
"asset_to_buy": "1.3.330",
"asset_to_buy_issuer": "1.2.3563",
"markets": ["1.3.329"]},

Problems are

* This extension requires the `owner` and `active` to be the `null-account`
* If you want max usability, you want to be able to buy back `X` with assets `A`, `B`, ... which is possible already, but the operation fails to pay for the fee because `BTS`/`TEST` are not part of `markets`. If it was, the core asset would be used to buy back `X`.

**Proposal**: Allow transfers of `CORE` to a special account to pay for the order creation fee **IFF** `CORE` is not listed in `markets`.

Discuss!

Begin with the account_create buyback_options extension and the order-creation fee path. Clarify the proposed CORE transfer behavior, including the null-account ownership constraint and the case where CORE is absent from markets; the issue is ready only once the behavior is agreed and validated.

Written by the indexing model from the issue text.