The existing `acme.TLSALPN01` challenge class did not have
a `response_cls`, meaning it was not possible to use a tls-alpn-01
challenge with `client.answer_challenge`.
To support the above a simple `TLSALPN01Response` class is added that
doesn't provide the ability to solve a tls-alpn-01 challenge end to end
(e.g. generating and serving the correct response certificate) but that
does allow the challenge to be initiated. This is sufficient for users
that have set up the challenge response independent of the `acme`
module code.
Resolves#6676
