withdraw destination should be in core and mint too

[ohaddahan]

https://github.com/ChainDev931105/anchor-coin-flip/blob/8f39241e7891a2350cc886ce95fb67d9d1b19f71/programs/coin-flip/src/lib.rs#L443-L448

we want to make each core use a specific mint, to avoid a mess of people trying to bet with various unrelated mints.

the withdraw destination should be predefined doing creation, to ensure more security => can add an update instruction to all of these for flexibility

[ChainDev931105]

yes, only registered mint is possible for users to bet function register will be called only by admin