Support `getVersionInfo` in the RPC client (`rpc.Server`)

[Shaptic]

The getVersionInfo RPC call was introduced in https://github.com/stellar/soroban-rpc/pull/132 with the following API response:

interface Response {
    version: string;
    commit_hash: string;
    build_time_stamp: string;
    captive_core_version: string;
    protocol_version: number; // uint32
}

Notice that the fields are underscored rather than the camelCase for the other endpoints. This was a mistake on release and will be rectified when Protocol 22 is released by https://github.com/stellar/soroban-rpc/pull/164 to avoid breaking changes in the meantime.