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/config/testdata/bad/multiple_expression_types.json

@@ -0,0 +1,17 @@
+{
+  "bots": [
+    {
+      "name": "multiple-expression-types",
+      "action": "ALLOW",
+      "expression": {
+        "all": [
+          "userAgent.startsWith(\"git/\") || userAgent.contains(\"libgit\")",
+          "\"Git-Protocol\" in headers && headers[\"Git-Protocol\"] == \"version=2\"\n"
+        ],
+        "any": [
+          "userAgent.startsWith(\"evilbot/\")"
+        ]
+      }
+    }
+  ]
+}

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

@@ -0,0 +1,10 @@
+bots:
+- name: multiple-expression-types
+  action: ALLOW
+  expression:
+    all:
+    - userAgent.startsWith("git/") || userAgent.contains("libgit")
+    - >
+      "Git-Protocol" in headers && headers["Git-Protocol"] == "version=2"
+    any:
+    - userAgent.startsWith("evilbot/")