Update HIP-904: AccountCreateTransaction to allow maxAutomaticTokenAssociations value to support -1 as a valid value

[SimiHunjan]

• The value -1 is now permitted for max_auto_associations, and this value means “unlimited”.
• The account holder no longer prepays for unused slots regardless of the setting for max_auto_associations. Slots are only paid for when used, and are initially paid for by the sender during automatic associations. The account is responsible for covering rent for each association after the first auto-renewal period. used_auto_associations is incremented every time an auto-association is made.
• The default value for max_auto_associations for new automatically-created accounts will now be -1. This means, if an account is created automatically (by performing a token transfer to an alias that does not yet exist), then the automatically created account is configured with unlimited automatic token associations. Accounts created by using the AccountCreateTransaction will continue to have a max_auto_associations default of 0. Accounts created before this feature is released will remain unchanged.