Cross-compatible hook/middleware + "plugins" which are allowed to be hook and middleware

[brandur]

This one's largely aimed at extending a few parts of otelriver to be
able to emit some additional useful metrics like time that it takes to
lock jobs, number of jobs locked per batch, or any other arbitrary
metrics we want to emit down the road. I previously had something
similar back in #1203, but here we extract an isolated piece of it.

This change does two things:

• If either a hook sent to Config.Hooks implements a middleware or a
  middleware sent to Config.Middleware implements a hook, activate its
  alternate side as well.

• Establish a new Config.Plugins that acts as a more generalized place
  where a hook/middleware can go. We define a plugin as this type:

    type Plugin interface {
        Hook
        Middleware
    }
  

The reason for the first point is better backward compatibility.
Notably, if I add a hook to otelriver.Middleware, I want it to be able
to still work even if the user doesn't explicitly movie it from
Config.Middleware to Config.Plugins.