Allow usage with api only apps

[bbarberBPL]

Currently if you are using this gem in an app with config.api_only = true and require 'sprockets/railtie' commented out in config/application.rb` you get the following error message when starting the server

../.rbenv/versions/2.7.6/lib/ruby/gems/2.7.0/gems//blacklight_oai_provider-7.0.2/lib/blacklight_oai_provider/engine.rb:9:in `block in <class:Engine>'
../.rbenv/versions/2.7.6/lib/ruby/gems/2.7.0/gems/railties-6.1.6/lib/rails/railtie/configuration.rb:97:in `method_missing': undefined method `assets' for #<Rails::Application::Configuration:0x0000562069563760> (NoMethodError)

This is due to this line. https://github.com/projectblacklight/blacklight_oai_provider/blob/7728dba9a38ceb6165335dea817b23c416d82966/lib/blacklight_oai_provider/engine.rb#L8
Adding a check before this to determine if the app is running in API only mode should fix this.