feat(checker): add CEL for matching complicated expressions (#421)

* feat(lib/policy): add support for CEL checkers

This adds the ability for administrators to use Common Expression
Language[0] (CEL) for more advanced check logic than Anubis previously
offered.

These can be as simple as:

```yaml
- name: allow-api-routes
  action: ALLOW
  expression:
    and:
    - '!(method == "HEAD" || method == "GET")'
    - path.startsWith("/api/")
```

or get as complicated as:

```yaml
- name: allow-git-clients
  action: ALLOW
  expression:
    and:
    - userAgent.startsWith("git/") || userAgent.contains("libgit") || userAgent.startsWith("go-git") || userAgent.startsWith("JGit/") || userAgent.startsWith("JGit-")
    - >
      "Git-Protocol" in headers && headers["Git-Protocol"] == "version=2"
```

Internally these are compiled and evaluated with cel-go[1]. This also
leaves room for extensibility should that be desired in the future. This
will intersect with #338 and eventually intersect with TLS fingerprints
as in #337.

[0]: https://cel.dev/
[1]: https://github.com/google/cel-go

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

* feat(data/apps): add API route allow rule for non-HEAD/GET

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

* docs: document expression syntax

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

* fix: fixes in review

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

---------

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

lib/policy/celchecker.go

@@ -0,0 +1,108 @@
+package policy
+
+import (
+	"fmt"
+	"net/http"
+
+	"github.com/TecharoHQ/anubis/internal"
+	"github.com/TecharoHQ/anubis/lib/policy/config"
+	"github.com/TecharoHQ/anubis/lib/policy/expressions"
+	"github.com/google/cel-go/cel"
+	"github.com/google/cel-go/common/types"
+)
+
+type CELChecker struct {
+	src     string
+	program cel.Program
+}
+
+func NewCELChecker(cfg *config.ExpressionOrList) (*CELChecker, error) {
+	env, err := expressions.NewEnvironment()
+	if err != nil {
+		return nil, err
+	}
+
+	var src string
+	var ast *cel.Ast
+
+	if cfg.Expression != "" {
+		src = cfg.Expression
+		var iss *cel.Issues
+		interm, iss := env.Compile(src)
+		if iss != nil {
+			return nil, iss.Err()
+		}
+
+		ast, iss = env.Check(interm)
+		if iss != nil {
+			return nil, iss.Err()
+		}
+	}
+
+	if len(cfg.All) != 0 {
+		ast, err = expressions.Join(env, expressions.JoinAnd, cfg.All...)
+	}
+
+	if len(cfg.Any) != 0 {
+		ast, err = expressions.Join(env, expressions.JoinOr, cfg.Any...)
+	}
+
+	if err != nil {
+		return nil, err
+	}
+
+	program, err := expressions.Compile(env, ast)
+	if err != nil {
+		return nil, fmt.Errorf("can't compile CEL program: %w", err)
+	}
+
+	return &CELChecker{
+		src:     src,
+		program: program,
+	}, nil
+}
+
+func (cc *CELChecker) Hash() string {
+	return internal.SHA256sum(cc.src)
+}
+
+func (cc *CELChecker) Check(r *http.Request) (bool, error) {
+	result, _, err := cc.program.ContextEval(r.Context(), &CELRequest{r})
+
+	if err != nil {
+		return false, err
+	}
+
+	if val, ok := result.(types.Bool); ok {
+		return bool(val), nil
+	}
+
+	return false, nil
+}
+
+type CELRequest struct {
+	*http.Request
+}
+
+func (cr *CELRequest) Parent() cel.Activation { return nil }
+
+func (cr *CELRequest) ResolveName(name string) (any, bool) {
+	switch name {
+	case "remoteAddress":
+		return cr.Header.Get("X-Real-Ip"), true
+	case "host":
+		return cr.Host, true
+	case "method":
+		return cr.Method, true
+	case "userAgent":
+		return cr.UserAgent(), true
+	case "path":
+		return cr.URL.Path, true
+	case "query":
+		return expressions.URLValues{Values: cr.URL.Query()}, true
+	case "headers":
+		return expressions.HTTPHeaders{Header: cr.Header}, true
+	default:
+		return nil, false
+	}
+}