add endpoint to initialize user

[wphan]

see response from #5

User should be able to initialize a user from gateway:

• if no exists, there'll be an RPC error finding the account (issue Surface more RPC errors to user #5 ), should prompt user to initialize their drift user
• If initializing the first drift user (subaccountId = 0), need to also initialize the userStats (ts sdk)
• otherwise to create more subaccounts just pass subAccountId > 0