Create a message to migrate ERC20Custody funds

[lumtis]

Would be better to address #2277 first

Describe the Issue
Create a message, let's call it MsgMigrateERC20CustodyFunds, to keep some consistency with MsgMigrateTSSFunds that allows to instruct the TSS to use withdraw method of ERC20Custody to migrate a specific asset
To be group admin message

type MsgMigrateERC20CustodyFunds {
  creator
  chainID
  asset
  amount
}

Note: I think the message should be in observer, and we should later consider moving MsgMigrateTSSFunds to observer module as well