feat(config): custom weight thresholds via CEL (#688)

* feat(config): add Thresholds to the top level config file

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

* chore(config): make String() on ExpressionOrList join the component expressions

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

* test(config): ensure unparseable json fails

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

* fix(config): if no thresholds are set, use the default thresholds

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

* feat(policy): half implement thresholds

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

* chore(policy): continue wiring things up

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

* feat(lib): wire up thresholds

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

* test(lib): handle behavior from legacy configurations

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

* docs: document thresholds

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

* docs: update CHANGELOG, refer to threshold configuration

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

* fix(lib): fix build

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

* chore(lib): fix U1000

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

---------

Signed-off-by: Xe Iaso <me@xeiaso.net>
Signed-off-by: Jason Cameron <git@jasoncameron.dev>
Co-authored-by: Jason Cameron <git@jasoncameron.dev>

lib/policy/config/testdata/bad/unparseable.json

@@ -0,0 +1 @@
+}

lib/policy/config/testdata/bad/unparseable.yaml

@@ -0,0 +1 @@
+}

lib/policy/config/testdata/good/no-thresholds.yaml

@@ -0,0 +1,8 @@
+bots:
+  - name: simple-weight-adjust
+    action: WEIGH
+    user_agent_regex: Mozilla
+    weight:
+      adjust: 5
+
+thresholds: []

lib/policy/config/testdata/good/thresholds.yaml

@@ -0,0 +1,38 @@
+bots:
+  - name: simple-weight-adjust
+    action: WEIGH
+    user_agent_regex: Mozilla
+    weight:
+      adjust: 5
+
+thresholds:
+  - name: minimal-suspicion
+    expression: weight < 0
+    action: ALLOW
+  - name: mild-suspicion
+    expression:
+      all:
+        - weight >= 0
+        - weight < 10
+    action: CHALLENGE
+    challenge:
+      algorithm: metarefresh
+      difficulty: 1
+      report_as: 1
+  - name: moderate-suspicion
+    expression:
+      all:
+        - weight >= 10
+        - weight < 20
+    action: CHALLENGE
+    challenge:
+      algorithm: fast
+      difficulty: 2
+      report_as: 2
+  - name: extreme-suspicion
+    expression: weight >= 20
+    action: CHALLENGE
+    challenge:
+      algorithm: fast
+      difficulty: 4
+      report_as: 4