feat: implement challenge registry (#607)

* feat: implement challenge method registry

This paves the way for implementing a no-js check method (#95) by making
the challenge providers more generic.

Signed-off-by: Xe Iaso <me@xeiaso.net>

* fix(lib/challenge): rename proof-of-work package to proofofwork

Signed-off-by: Xe Iaso <me@xeiaso.net>

* fix(lib): make validated challenges a CounterVec

Signed-off-by: Xe Iaso <me@xeiaso.net>

* fix(lib): annotate jwts with challenge method

Signed-off-by: Xe Iaso <me@xeiaso.net>

* test(lib/challenge/proofofwork): implement tests

Signed-off-by: Xe Iaso <me@xeiaso.net>

* test(lib): add smoke tests for known good and known bad config files

Signed-off-by: Xe Iaso <me@xeiaso.net>

* docs: update CHANGELOG

Signed-off-by: Xe Iaso <me@xeiaso.net>

* fix(lib): use challenge.Impl#Issue when issuing challenges

Signed-off-by: Xe Iaso <me@xeiaso.net>

---------

Signed-off-by: Xe Iaso <me@xeiaso.net>

lib/testdata/invalid-challenge-method.yaml

@@ -0,0 +1,8 @@
+bots:
+  - name: generic-bot-catchall
+    user_agent_regex: (?i:bot|crawler)
+    action: CHALLENGE
+    challenge:
+      difficulty: 16
+      report_as: 4
+      algorithm: hunter2 # invalid algorithm