Register OADs globally

[ahx]

Add OpenapiFirst#Configuration.register to register OADs globally.

This removes the necessity to load the OAD in the same place where you use the middlewares and adds a cache for parsed OADs.

# initializer.rb
OpenapiFirst.configure do |config|
  config.register('openapi/openapi.yaml')
end

# app.rb
use OpenapiFirst::Middlewares::RequestValidation

You currently still need to register OADS for which you want to track API coverage. I am not 100% sure if or how that should be changed.