Cashaccount wallet support.

I'd like to request integration for features for supporting cashaccounts on light wallets. Essentially an optional indexer with related API calls for lookup (account->payment data) and reverse lookup (payment data->account).

There exist an approved BUIP for this work within BU so by cooperating with them on the implementation could offset implementation cost and solidify the format used across nodes.

https://bitco.in/forum/threads/buip118-passed-implement-cashaccount-lookup-features.23749/