This is the interface for storing miner pre-exits, after calling this interface, you need to wait one day to withdraw your staking.
// MinerExitPrep pre-exit storage miner
//
// Return:
// - string: block hash
// - error: error message
//
// Note:
// - after pre-exit, you need to wait for one day before it will automatically exit
// - cannot register as a storage miner again after pre-exit
func (c *ChainClient) MinerExitPrep() (string, error)
