Make HsDescBuilder take certs as arguments, not generate them internally

The following discussion from !1615 (merged) should be addressed:

• @nickm started a discussion: (+1 comment)

  (IIRC we talked about how it makes more sense to take the certs as arguments, but that can happen later.)

HsDescBuilder generates the intro_auth_key_cert and intro_enc_key_cert internally, using the expiry values specified via intro_auth_key_cert_expiry()/intro_enc_key_cert_expiry(). Perhaps the builder shouldn't be generating these certs at all (instead of taking the cert expiry as an argument, it should take the intro_auth_key_cert/intro_enc_key_cert, which would be generated by the caller).