Authenticator and PrivateKey should not be required by default

Authenticator function and private key are required now if you want to use this middleware in a Gin project. But sometimes your application does not issue JWT-tokens, refresh and destroy them (especially in a microservice architecture where this functions usually do a separate service), but only needs an ability to read the token and extract claims from it. In such a case you just should provide a public file and encryption algorithm name. For example, Echo framework jwt-middleware could be created with only public key and algo name (see https://echo.labstack.com/middleware/jwt/):
```
jwtConfig := middleware.JWTConfig{
SigningKey: publicKey,
SigningMethod: jwt.SigningMethodRS256.Name,
}
```
I believe that this middleware also should require only minimum for tokens parsing.

Start by tracing the middleware configuration and token-parsing path, focusing on how Authenticator, private key, public file, and encryption algorithm are validated. Done means a Gin application can omit Authenticator and private key while providing a public key and algorithm to read tokens and extract claims, without breaking existing token-issuing behavior.

Written by the indexing model from the issue text.