Support metadata from middleware

[StoneFrog]

Currently, we store job args, class_name etc. coming by default with sidekiq.
In some cases, we may want to use additional middleware to pass over, for example, correlation_id or causation_id. While this will work in regular flow, we will lose it in case there is some crash and the job is removed from Redis. In such case, MissedJobsScheduler, will execute those jobs fine, yet without middleware data.

The idea would be to persist context from custom middleware either as a config with keys to store, or convention (eg. make middlewares add all metadata under given key and make robust gem use it)