Get an array of all unspent coins for an AccountIdentifier and the
BlockIdentifier at which the lookup was performed. If your implementation
does not support coins (i.e. it is for an account-based blockchain),
you do not need to implement this endpoint. If you implementation does
support coins (i.e. it is fro a UTXO-based blockchain), you MUST
also complete the /account/balance endpoint.
It is important to note that making a coins request for an account
without populating the SubAccountIdentifier should not result in the
coins of all possible SubAccountIdentifiers being returned. Rather,
it should result in the coins pertaining to no SubAccountIdentifiers
being returned. To get all coins associated with an account, it may be
necessary to perform multiple coin requests with unique AccountIdentifiers.
Optionally, an implementation may choose to support updating an AccountIdentifier's
unspent coins based on the contents of the mempool. Note, using this functionality
breaks any guarantee of idempotency.