Features `experimental-api` and `onion-service-client` are incompatible

Summary

arti-client does not compile with the experimental-api and onion-service-client features unless some other (non-default) feature is enabled that happens to include tor-hsservice as a dependency for the arti-client crate.

Steps to reproduce:

cargo check -p arti-client --features experimental-api,onion-service-client

What is the current bug behavior?

error[E0432]: unresolved import `tor_hsservice`
  --> crates/arti-client/src/client.rs:47:60
   |
47 |     tor_hscrypto::pk::HsId, tor_hscrypto::pk::HsIdKeypair, tor_hsservice::HsIdKeypairSpecifier,
   |                                                            ^^^^^^^^^^^^^ use of undeclared crate or module `tor_hsservice`

Possible fixes:

While we only need this if both the experimental-api and onion-service-client features are enables, I don't think Cargo lets us specify that, so we should probably fix this by adding tor-hsservice as a dependency of the onion-service-client feature.