Draft: Add HS client key manager sketch.

• Add the key manager config to the example configs.

• Make TorClient use HsClientKeyMgr.

  • TorClient now contains a HsClientKeyMgr for retrieving client auth keys

  • arti-client has some new *Config types for configuring a HsClientKeyMgr that uses either the legacy storage format or the current one (TBD what the current one looks like)

• Add HS client key manager sketch.

  This is a first attempt at defining an API for the HS client key manager.

  • the client key manager (HsClientKeyMgr) lives in the new tor-keymgr crate. Future versions of this crate will also expose HsServiceKeyMgr, which will be used by services to manage client auth keys
  • the tor-hsclient::keys module is moved to tor-keymgr::keys (to avoid a circular dependency between tor-hsclient and tor-keymgr)

• Fix typo in HS auth key notes.

  The only type of client authorization key used in C Tor today is KS_hsc_client_desc_enc.

Part of #728 (closed).