feat(auth): public list authentication methods endpoint

[GeorgeMac]

Fixes FLI-120

Implements the /auth/v1/method endpoint.
This endpoint lists metadata about which authentication methods are configured and in what way.

I have first introduced a small refactor to the config/authentication related config.
This allows for simple registering of new methods, while ensuring they are then appropriately reflected in the API without additional changes.

I also added some cases to the e2e test, which invokes this endpoint to ensure it's mounted.
And another which attempts to use the Cookie header to authenticate a request when OIDC is enabled.

curl http://localhost:8080/auth/v1/method | jq .                                                                                                                                              
{
  "methods": [
    {
      "method": "METHOD_TOKEN",
      "enabled": false,
      "sessionCompatible": false
    },
    {
      "method": "METHOD_OIDC",
      "enabled": false,
      "sessionCompatible": true
    }
  ]
}

[markphelps]

do you think its any kind of security risk to publicize all the methods, even if they arent enabled? I could potentially see a situation where an attacker could leverage this information somehow (maybe?) create an exploit?

do you think its worth only returning enabled methods?

[markphelps]

i mean i guess since its open source, and has public docs that we arent really able to hide which methods are available

[markphelps]

we'll need to update https://github.com/flipt-io/flipt-api once we release