RFC for account balances

[gouthamp-stellar]

Write an RFC for a feature that will allow wallets to fetch the balances of every token that an account holds. Wallets should be able to specify all the tokens for an accouns that it wants to track balances for and retrieve these token balances via an API call. NOTE: We have also have to ensure that the account balances returned by the wallet backend match the account balances that we would've otherwise gotten from RPC

Link to spike ticket: https://github.com/orgs/stellar/projects/58/views/2?pane=issue&itemId=91241787&issue=stellar%7Cwallet-backend%7C91