Allow client to query application balances on chains

[jvff]

Motivation

Right now, it's possible to query the chain balance and the authenticated signer's balance by staging a block execution. However, it's not possible to query the balance of an application.

Proposal

Support querying the balance of an application's account in the client, possibly by a separate query to the local node from linera_core::ChainClient::stage_block_execution_and_discord_failing_messages.