Feature: add schema registry authentication settings

### Feature description

I've just realized that schema registry doesn't support auth settings.

It would be great to have extra options like:
```
--schema-registry.confluent.connection-string "redpanda-endpoint:8081"
--schema-registry.type confluent
--schema-registry.authentication.type basic
--schema-registry.authentication.basic.username "username"
--schema-registry.authentication.basic.password "password"
```

The reason why I suggest authentication parameters to be 'outside' confluent is because both redpanda and apicurio implement confluent protocol, but add authentication on top of it.