caddytls: introduced own ChallengeProvider type to fix imports related to vendor (#1700)
caddytls: introduced own ChallengeProvider type to fix imports related to vendor (#1700)

* introduced own ChallengeProvider type, based on acme.ChallengeProvider to avoid vendoring/version mismatches in Caddy plugins; see Caddy issue #1697

* fixed up comments for ChallengeProvider

* moved ChallengeProvider to caddytls/tls.go

captncraig/caddy,cez81/caddy,yukimochi/caddy,cez81/caddy,captncraig/caddy,dusty/caddy,devangels/caddy,dusty/caddy,joshix/caddy,tobya/caddy,yukimochi/caddy,tobya/caddy,dusty/caddy,captncraig/caddy,joshix/caddy,yukimochi/caddy,dusty/caddy,cez81/caddy,tobya/caddy,devangels/caddy,joshix/caddy,tobya/caddy,devangels/caddy