Wasm execution on embedded client should be configurable

The embedded client runs in `Interpreted` mode which is quite slow (at least in [our benchmarks](https://gitlab.com/interlay/btc-parachain/-/tree/dev/crates/issue#benchmarks)). [This](https://github.com/paritytech/substrate-subxt/blob/master/client/src/lib.rs#L286) should be configurable, perhaps via a `wasmtime` cfg feature.

