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/challenge/error.go

@@ -0,0 +1,37 @@
+package challenge
+
+import (
+	"errors"
+	"fmt"
+	"net/http"
+)
+
+var (
+	ErrFailed        = errors.New("challenge: user failed challenge")
+	ErrMissingField  = errors.New("challenge: missing field")
+	ErrInvalidFormat = errors.New("challenge: field has invalid format")
+)
+
+func NewError(verb, publicReason string, privateReason error) *Error {
+	return &Error{
+		Verb:          verb,
+		PublicReason:  publicReason,
+		PrivateReason: privateReason,
+		StatusCode:    http.StatusForbidden,
+	}
+}
+
+type Error struct {
+	Verb          string
+	PublicReason  string
+	PrivateReason error
+	StatusCode    int
+}
+
+func (e *Error) Error() string {
+	return fmt.Sprintf("challenge: error when processing challenge: %s: %v", e.Verb, e.PrivateReason)
+}
+
+func (e *Error) Unwrap() error {
+	return e.PrivateReason
+}