Support subaddress lookahead in monero-wallet-rpc
This issue requests setting the wallet's subaddress lookahead with monero-wallet-rpc.

Preferably a new RPC call is added, e.g. `set_subaddress_lookahead`, but it needs to be confirmed if that works after the initial wallet is created, since most or all times `wallet2::set_subaddress_lookahead()` is used, it's before `wallet2::generate()` is called. For example: https://github.com/monero-project/monero/blob/00fd416a99686f0956361d1cd0337fe56e58d4a7/src/simplewallet/simplewallet.cpp#L4658

If it turns out the subaddress lookahead must be set when the wallet is created, a new `subaddress_lookahead` parameter can be added to these methods in wallet_rpc_server.cpp:

- `create_wallet`
- `restore_deterministic_wallet`
- `generate_from_keys`