Add support for custom JSONSchemer configuration

[moberegger]

A small change to make it so that the global JSONSchemer.configuration is respected when openapi_first builds its router. I made it so that openapi_first will first clone the configuration before making the changes it needs to so that it avoids adjusting the global configuration in case something else in a dev's project is using it.

This useful for something like supporting custom formats. For example, I can now do something like

JSONSchemer.configure do |config|
  config.formats = { 'uuid' => ->(instance, format) { Utils.is_uuid?(instance) } }
end

OpenapiFirst.load('openapi.yaml')

and openapi_first will pick up that configuration and apply it to the JSONSchemer.schemas it creates as it builds the router, which allows it to work with my custom format.

Prior to version 2.2, this would work. Recent changes made it so that a new JSONSchemer::Configuration instance was created, so any prior changes made to JSONSchemer.configuration would no longer be applied.

[ahx]

Can you add a test for this so we avoid breaking this (again) in the future?

[ahx]

I am struggling with finding a place to test this, but I just added something on another branch I will merge this to.
Thank you. This seems super useful.