Use caddy's certmagic library for extensible/robust ACME handling (#14177)
* use certmagic for more extensible/robust ACME cert handling
* accept TOS based on config option
Signed-off-by: Andrew Thornton <art27@cantab.net>
Co-authored-by: zeripath <art27@cantab.net>
Co-authored-by: Lauris BH <lauris@nix.lv>