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>

vendor/go.uber.org/multierr/CHANGELOG.md

@@ -0,0 +1,54 @@
+Releases
+========
+
+v1.5.0 (2020-02-24)
+===================
+
+-   Drop library dependency on development-time tooling.
+
+
+v1.4.0 (2019-11-04)
+===================
+
+-   Add `AppendInto` function to more ergonomically build errors inside a
+    loop.
+
+
+v1.3.0 (2019-10-29)
+===================
+
+-   Switch to Go modules.
+
+
+v1.2.0 (2019-09-26)
+===================
+
+-   Support extracting and matching against wrapped errors with `errors.As`
+    and `errors.Is`.
+
+
+v1.1.0 (2017-06-30)
+===================
+
+-   Added an `Errors(error) []error` function to extract the underlying list of
+    errors for a multierr error.
+
+
+v1.0.0 (2017-05-31)
+===================
+
+No changes since v0.2.0. This release is committing to making no breaking
+changes to the current API in the 1.X series.
+
+
+v0.2.0 (2017-04-11)
+===================
+
+-   Repeatedly appending to the same error is now faster due to fewer
+    allocations.
+
+
+v0.1.0 (2017-31-03)
+===================
+
+-   Initial release