Add solidus static content content provider

[vassalloandrea]

Summary

Add solidus static content - content provider

To retrieve the page we have to pass the page slug to the entry options.
If the page slug is the same as the entry one, we can avoid passing the options.

posts = SolidusContent::EntryType.create(
  name: 'posts',
  content_provider_name: 'solidus_static_content'
)

entry = SolidusContent::Entry.create!(
  slug: '2020-03-27-hello-world',
  entry_type: posts,
  options: { slug: 'XXX' } # Can be omitted if the page slug is the same of the entry
)

[elia]

Added a suggestion on how to fix the CI