Add UTXO endpoint to webserver

[SebastienGllmt]

A Utxo endpoint would just be taking the TransctionOutput table minus all entries that appear in the TransactionInput table.

User would pass in addresses to the endpoint.

TBD: should we allow users to pass in credentials? UTXOs belong to addresses and not credentials so I was thinking that users would first call credential/addresses and then use those addresses to query the utxo endpoint. However, the endpoint could combine these two steps for users to avoid an extra round-trip