add nip-41: authentication

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+NIP-41
+======
+
+Authentication of clients to relays
+-----------------------------------
+
+`draft` `optional` `author:Semisol`
+
+This NIP defines a way for clients to authenticate to relays by signing an ephemeral event.
+
+## Event format
+
+An event should be signed with `kind: 22241` and `content` being the WebSockets URL of the relay.
+The URL MUST:
+- Have a trailing slash if the path is `/`
+- Not have a port number if protocol is `ws` and port is `80` or protocol is `wss` and port is `443`
+- Not include the search/query
+
+An example event is shown below:
+```json
+{
+  "id": "...",
+  "pubkey": "...",
+  "created_at": 1669695536,
+  "kind": 22223,
+  "tags": [],
+  "content": "wss://relay.example.com/",
+  "sig": "..."
+}
+```
+
+## Commands between the relay and the client
+
+This NIP defines a new command, `AUTH`, which can be sent by either the relay or the client with different meanings.
+
+A relay may send `["AUTH", <human readable reason>, <data object>]` when it needs authentication (examples: accessing kind 4 events, whitelist only, requiring proof of authorship for `EVENT`).  
+The human readable reason SHOULD be prefixed with a string in the format `<short desc string>: <description>`. A list of short descriptions is listed below:
+- `restricted`: This relay is restricted and requires the pubkey of the client to check if it can access the relay. (requires a whitelist, payment, etc)
+- `publish`: The relay requests that the client identify who is sending an `EVENT` command.
+  This can be used for where only the signer of an event can publish it, or a pay-as-you-go relay allowing for you to publish others' events 
+- `private`: The client has attempted to access a restricted set of events (example: kind 4 DMs) and should authenticate with the relay to receive them.
+- `other`: Any reason not defined here.
+
+`data object` MUST be a JSON object. It currently has one defined field, but may be extended by amendments to this NIP or other NIPs:
+- `subscription_id`: The subscription ID that triggered the `AUTH` request.
+
+A client may send one of the following to the relay:
+- `["AUTH", <signed event>]` to indicate it has accepted the request. This may also be sent without an authentication request.
+- `["AUTH", null]` to indicate it has rejected the request.
+
+A relay SHOULD send the [`OK`](https://github.com/nostr-protocol/nips/blob/master/20.md) command after they receive a
+non-rejecting authentication response, and use one of the following `message` prefixes if the event sent cannot be verified:
+- `too-old:`: The event signed has a too old `created_at` date.
+- `invalid-url:`: The URL in `content` is not matching.
+- `already-used:`: This event was already used to authenticate to this relay.
+- `bad-signature:`: The event has a bad signature.
+
+Please note that the `OK` message should only be sent as a response to other commands that the client sends instead of the `AUTH` command if the issue is not related to the authentication event being incorrectly signed (example: not on whitelist).
+
+Relays SHOULD send [`EOSE`](https://github.com/nostr-protocol/nips/blob/master/15.md) when an authentication request is triggered by a `REQ` command, and not send stored events after the `EOSE` when authentication is completed.
+Relays SHOULD send `OK` as a response when a command triggers authentication with the reason starting with `auth:`.
+
+Clients SHOULD retry the action (resending event, resubscribing) after they authenticate if they receive an `AUTH` request.
+
+## Signed event verification
+Relays when processing `AUTH` responses SHOULD verify the following before accepting the response:
+- that the `kind` is `22223`
+- that the event was recently signed (~10 minutes, by `created_at`)
+- that the `content` field matches the relay URL