Remove the PLUGINS__ configuration section

The users don't need to know or care whether what they are configuration is a plugin or not.

e.g. Instead of EVENTSTORE__PLUGINS__AUTOSCAVENGE__ENABLED we could have EVENTSTORE__AUTOSCAVENGE__ENABLED

The tradeoff (and the reason why it is like this in the first place) is that

• the server detects unknown configuration options. we would need a way for the plugins to inform the server of what options they support, or the server could ignore all nested options

• we'd have to ensure plugin options don't conflict with root level options (shouldn't be difficult)