Get an array of all AccountBalances for an AccountIdentifier and the
BlockIdentifier at which the balance lookup was performed. The BlockIdentifier
must always be returned because some consumers of account balance data need
to know specifically at which block the balance was calculated to
compare balances they compute from operations with the balance returned
by the node.

It is important to note that making a balance request for an account
without populating the SubAccountIdentifier should not result in the
balance of all possible SubAccountIdentifiers being returned. Rather,
it should result in the balance pertaining to no SubAccountIdentifiers
being returned (sometimes called the liquid balance). To get all
balances associated with an account, it may be necessary to
perform multiple balance requests with unique AccountIdentifiers.

It is also possible to perform a historical balance lookup (if the server
supports it) by passing in an optional BlockIdentifier.