doc: added documentation for offline withdrawal functionality

[NkwaTambe]

Description:

Implement and document the offline withdrawal functionality where both the client and agent authenticate transactions using account certificates. The client generates a signed transaction request, which is then confirmed by the agent and validated by the backend server. The backend verifies both certificates (client and agent), checks the transaction details, and ensures proper authorization before processing the withdrawal.

[nancymuyeh]

TransactionObjectJson = { ClientAccountID, amount, ClientAccountCert }
OfflineWithrawalRequest = {
header: AgentAccountCert, TransactionObjectJson,
body: AgentAcoountID
}