Refactor handling of CircTargets with unknown protocol capabilities

At present, we sometimes want to construct a CircTarget when we don't know the Protocols for the relay. This happens when we aren't building the CircTarget from a Relay object, or from a RouterDesc, but instead from a set of information that we might have taken from (say) the onion service protocol.

When we do this, we currently proceed as follows:

• If there is a Relay in our latest NetDir that matches the target Relay's ids, we use the Protocols from that Relay.
• Otherwise, we use the required-relay-protocols from the latest consensus directory, as the bare minimum.

There are two inconvenient aspects to this:

• The logic is duplicated. It appears in circtarget_from_pieces in tor-hsclient, and in establish_session in tor-hsservice.
• The logic is, likely, at too high a level. It would be better if we could ensure that our CircTargets were always built with a reasonable set of protocols somehow, and it seems too easy to forget to follow this pattern.