Add proxy_connection_token option to call connect proxy with token

[FZambia]

Proposed changes

Adding proxy_connection_token boolean option. When it's set to true:

1. Centrifugo starts proxying connection token to the configured connect proxy. Centrifugo does not check token in any way – fully relying on the app backend to do this.
2. On the app backend token can be validated and connection expiration set. At some point in the future getToken function on the client side will be called.
3. Users can provide updated token to client SDK and SDK sends it to Centrifugo.
4. Centrifugo understands that this is a token which should be proxied to the backend (because proxy_connection_token on) and sends refresh HTTP proxy request to the backend with new token. Backend validates it and returns new expiration time to Centrifugo.
5. Centrifugo sends refresh response to the client like it does with Centrifugo native JWT case.

So this is a workflow which allows delegating token validation to the app backend and at the same time inherit all the logic of Centrifugo native JWT. Application backend is fully responsible for token expiration checks.

Inside connect proxy handler application should:

1. Validate token, check signature, expiration time, etc. I.e. do all the token checks.
2. If token is valid – then return connect proxy result with user and expiration time (if needed).
3. If token expired then application should return error 109 token expired in connect proxy response.
4. If token invalid then application should return disconnect 3500 invalid token in connect proxy response.

Inside refresh proxy handler application should:

1. Validate token, check signature, expiration time, etc. I.e. do all the token checks. We also suggest to check that user received from Centrifugo matches user in token.
2. If token is valid – then return refresh proxy result with new expiration time (if needed).
3. If token expired then application should return "result": {"expired": true} in refresh proxy response.
4. If token invalid then application should return disconnect 3500 invalid token in refresh proxy response (TODO: this is not possible now, need to add this logic to the refresh proxy handler).

Relates centrifugal/centrifuge-js#216

[FZambia]

After considering a bit more: it works, but it mostly eliminates all the performance benefits of JWTs, so trying to solve the related issue in alternative way – fighting with root cause which prevents using native JWT mechanism of Centrifugo instead of trying to introduce custom token workflow.

[FZambia]

Closed in favour of #638