OptionalfeeBase fee in stroops (default BASE_FEE = "100").
Must match the API deployment, RPC and signing wallet.
Current sequence number of the caller account, as a decimal string.
Callers fetch this from Soroban RPC (server.getAccount(caller)) before
building. Keeping it as an input (rather than fetching inside the builder)
makes builders sync-friendly, RPC-free, and deterministic for snapshot tests.
OptionaltimeoutTx timeout in seconds (default 300).
Transaction source and authorizing wallet public key (G...).