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/environment.go

@@ -0,0 +1,45 @@
+package expressions
+
+import (
+	"github.com/google/cel-go/cel"
+	"github.com/google/cel-go/ext"
+)
+
+// NewEnvironment creates a new CEL environment, this is the set of
+// variables and functions that are passed into the CEL scope so that
+// Anubis can fail loudly and early when something is invalid instead
+// of blowing up at runtime.
+func NewEnvironment() (*cel.Env, error) {
+	return cel.NewEnv(
+		ext.Strings(
+			ext.StringsLocale("en_US"),
+			ext.StringsValidateFormatCalls(true),
+		),
+
+		// default all timestamps to UTC
+		cel.DefaultUTCTimeZone(true),
+
+		// Variables exposed to CEL programs:
+		cel.Variable("remoteAddress", cel.StringType),
+		cel.Variable("host", cel.StringType),
+		cel.Variable("method", cel.StringType),
+		cel.Variable("userAgent", cel.StringType),
+		cel.Variable("path", cel.StringType),
+		cel.Variable("query", cel.MapType(cel.StringType, cel.StringType)),
+		cel.Variable("headers", cel.MapType(cel.StringType, cel.StringType)),
+
+		// Functions exposed to CEL programs:
+	)
+}
+
+// Compile takes CEL environment and syntax tree then emits an optimized
+// Program for execution.
+func Compile(env *cel.Env, ast *cel.Ast) (cel.Program, error) {
+	return env.Program(
+		ast,
+		cel.EvalOptions(
+			// optimize regular expressions right now instead of on the fly
+			cel.OptOptimize,
+		),
+	)
+}