Add a transport property that just indicates that the transport is supported

We need to know whether it's possible to communicate with a given contact via removable drives. The presence of transport keys doesn't necessarily mean the contact supports the transport, as when adding a contact we derive keys for all transports we support, without knowing whether the contact also supports them.

For all existing transports we can use transport properties to check whether the contact supports the transport. But the removable drive transport doesn't have any properties.

Add a "supported" property for all transports, so that we can tell which transports a contact supports regardless of whether any properties are needed for establishing a connection.

We may also be able to use this property for key derivation when adding a contact, so that we only derive keys for transports both parties support.