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/thresholds.go

@@ -0,0 +1,47 @@
+package policy
+
+import (
+	"github.com/TecharoHQ/anubis/lib/policy/config"
+	"github.com/TecharoHQ/anubis/lib/policy/expressions"
+	"github.com/google/cel-go/cel"
+)
+
+type Threshold struct {
+	config.Threshold
+	Program cel.Program
+}
+
+func ParsedThresholdFromConfig(t config.Threshold) (*Threshold, error) {
+	result := &Threshold{
+		Threshold: t,
+	}
+
+	env, err := expressions.ThresholdEnvironment()
+	if err != nil {
+		return nil, err
+	}
+
+	program, err := expressions.Compile(env, t.Expression.String())
+	if err != nil {
+		return nil, err
+	}
+
+	result.Program = program
+
+	return result, nil
+}
+
+type ThresholdRequest struct {
+	Weight int
+}
+
+func (tr *ThresholdRequest) Parent() cel.Activation { return nil }
+
+func (tr *ThresholdRequest) ResolveName(name string) (any, bool) {
+	switch name {
+	case "weight":
+		return tr.Weight, true
+	default:
+		return nil, false
+	}
+}