Handle invalid ext_authz request

[guicassolato]

Improved handler for invalid ext_authz requests errors.

Verification steps

make cluster install run

In a separate shell: (Requires grpcurl)

grpcurl -plaintext -d @ localhost:50051 envoy.service.auth.v3.Authorization.Check <<EOF
{}
EOF

Before:

Authorino panics.

After:

Authorino handles the error nicely and the returns the following response:

{
  "status": {
    "code": 3
  },
  "deniedResponse": {
    "status": {
      "code": "BadRequest"
    },
    "headers": [
      {
        "header": {
          "key": "X-Ext-Auth-Reason",
          "value": "Invalid request"
        }
      }
    ]
  }
}

[alexsnaps]

No related to this PR, just my curiosity... is this recognized by the compiler as a constant expression and optimized away? Or do we really end up with a HashMap on the heap?

[guicassolato]

I would imagine it's implemented as a hashmap on the heap, but really not sure.

Otherwise, the compiler would have to ensure there's no writing to it despite declared as var, but I guess offering var and const is precisely the way it has to say "it's your problem, not mine."

My only comfort is that it's a map of known size upfront. Performance should be acceptable (somewhat near O(1) in Golang.) Of course, not better than a lookup at compilation time.