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>

data/common/allow-api-like.yaml

@@ -0,0 +1,6 @@
+- name: allow-api-routes
+  action: ALLOW
+  expression:
+    all:
+    - '!(method == "HEAD" || method == "GET")'
+    - path.startsWith("/api/")

data/common/json-api.yaml

@@ -0,0 +1,7 @@
+- name: allow-api-requests
+  action: ALLOW
+  expression:
+    all:
+      - '"Accept" in headers'
+      - 'headers["Accept"] == "application/json"'
+      - 'path.startsWith("/api/")'

data/common/keep-internet-working.yaml

@@ -7,4 +7,7 @@
   action: ALLOW
 - name: robots-txt
   path_regex: ^/robots.txt$
+  action: ALLOW
+- name: sitemap
+  path_regex: ^/sitemap.xml$
   action: ALLOW

data/common/rfc-violations.yaml

@@ -0,0 +1,3 @@
+- name: no-user-agent-string
+  action: DENY
+  expression: userAgent == ""