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/expressions/url_values.go

@@ -0,0 +1,78 @@
+package expressions
+
+import (
+	"errors"
+	"net/url"
+	"reflect"
+	"strings"
+
+	"github.com/google/cel-go/common/types"
+	"github.com/google/cel-go/common/types/ref"
+	"github.com/google/cel-go/common/types/traits"
+)
+
+var ErrNotImplemented = errors.New("expressions: not implemented")
+
+// URLValues is a type wrapper to expose url.Values into CEL programs.
+type URLValues struct {
+	url.Values
+}
+
+func (u URLValues) ConvertToNative(typeDesc reflect.Type) (any, error) {
+	return nil, ErrNotImplemented
+}
+
+func (u URLValues) ConvertToType(typeVal ref.Type) ref.Val {
+	switch typeVal {
+	case types.MapType:
+		return u
+	case types.TypeType:
+		return types.MapType
+	}
+
+	return types.NewErr("can't convert from %q to %q", types.MapType, typeVal)
+}
+
+func (u URLValues) Equal(other ref.Val) ref.Val {
+	return types.Bool(false) // We don't want to compare header maps
+}
+
+func (u URLValues) Type() ref.Type {
+	return types.MapType
+}
+
+func (u URLValues) Value() any { return u }
+
+func (u URLValues) Find(key ref.Val) (ref.Val, bool) {
+	k, ok := key.(types.String)
+	if !ok {
+		return nil, false
+	}
+
+	if _, ok := u.Values[string(k)]; !ok {
+		return nil, false
+	}
+
+	return types.String(strings.Join(u.Values[string(k)], ",")), true
+}
+
+func (u URLValues) Contains(key ref.Val) ref.Val {
+	_, ok := u.Find(key)
+	return types.Bool(ok)
+}
+
+func (u URLValues) Get(key ref.Val) ref.Val {
+	result, ok := u.Find(key)
+	if !ok {
+		return types.ValOrErr(result, "no such key: %v", key)
+	}
+	return result
+}
+
+func (u URLValues) Iterator() traits.Iterator { panic("TODO(Xe): implement me") }
+
+func (u URLValues) IsZeroValue() bool {
+	return len(u.Values) == 0
+}
+
+func (u URLValues) Size() ref.Val { return types.Int(len(u.Values)) }