Allow overriding tunnel server protocol/domain for caBLE

[micolous]

This adds a cable-override-tunnel feature.

This allows the library to override which caBLE tunnel server protocol and domain it connects to, rather than the actual tunnel server. This is useful for testing #291 (caBLE tunnel server implementation), so one can run a tunnel server locally without needing to set up DNS and certificates.

Example

To act as an initiator overriding the tunnel server to be at ws://localhost:8080:

cargo run \
  --example authenticate \
  --features cable,cable-override-tunnel \
  -- \
  cable --tunnel-uri ws://localhost:8080

To act as an authenticator in the same way:

cargo run \
  --example cable_tunnel \
  --features cable,cable-override-tunnel \
  -- \
  --serial-port /dev/tty.usbmodem \
  --softtoken-path /tmp/softtoken.dat \
  --qr-image ~/Desktop/screenshot.png \
  --tunnel-uri ws://localhost:8080

Changes

Related fixing (which blocked this change):

• only run doctest for features that are enabled

Public API changes:

• share_cable_authenticator()'s parameters changed:

  • added options: ShareCableAuthenticatorOptions parameter
  • tunnel_server_id argument moved to ShareCableAuthenticatorOptions
  • close_after_one_command argument inverted to stay_open_after_one_command, and moved to ShareCableAuthenticatorOptions
  • added tunnel_uri field to ShareCableAuthenticatorOptions

• Added connect_cable_authenticator_with_tunnel_uri method for new feature.

  This will probably be changed to use a ConnectCableAuthenticatorOptions struct like ShareCableAuthenticatorOptions once there's something else to put in there.

Internal API changes:

• Eid::get_domain replaced with Eid::get_domain_builder
• Eid::get_connect_uri now (internally) public
• Added Eid::build_connect_uri (internally public)
• Added Discovery::build_new_tunnel_uri (internally public)
• Discovery::get_connect_uri removed (use Eid::get_connect_uri instead)
• Added uri parameter to Tunnel::connect_authenticator

• cargo test has been run and passes
• documentation has been updated with relevant examples (if relevant)