Enhance `OfflineClient` capabilities with unilateral exit

[ethicnology]

Currently OfflineClient can:

• connect
• connect_with_retries

I would expect the OfflineClient to also be able to:

• get the available balance for unilateral exit
• unilaterally exit
• get Ark address
• get Boarding address

Some of these features may require the server's keys. Retrieving server information could be made available separately, without the needing to initialize a client.