feat(cardano): allow reading of UTXOs by an address

[carbolymer]

In cardano we are able to query UTXOs by TxIn (txoRef in u5c) or by an address. This PR adds capability of querying by addresses to proto definition.

There's also an open question:
What should a server implementation do when an empty list of tx output references or addresses is provided? Should a recommendation for that case be placed in the comments?
It would be convenient to just return a whole UTXO set for that case, which would be very useful for testing.

[scarmuega]

@carbolymer the idea of the SearchUtxos call is to solve the needs for querying utxos in a flexible way where complex filters can be specified using different patterns (including addresses).

In contrast, the ReadUtxos call is meant to be a direct way of getting utxos in a key-value fashion, that's why it's constrained to TxoRef (our key in this case).

We decided to split it in this way since there are several optimizations (caching, batch loading, etc) that can be performed only when doing a key-value retrieval.

Do you think that the SearchUtxos method fulfills your requirements? or is there any reason to modify the ReadUtxos call?

[scarmuega]

as discussed in the bi-weekly, the guideline for this use-case is to use the search utxos endpoints.