Create ProvidetType for BlogPost

Create a ProviderType for BlogPost. A typical BlogPostProvider should be able to do the following:

• Get a total count of available posts.
• Receive the n newest posts, cronologically (publishdate), not by when they were added.
• Receive post n to m.
• The two points above should be able to do sow in the well-known syntax SomeGetFunc(int pageIndex, int pageSize, out int totalPosts).
• Get posts by a list of ids.
• Add post.
• Delete post.
• Modify post.

A post should at least contain the following information:

• The id of the post.
• The title of the post.
• The slug of the post.
• The Ingress-part of the post (might need renaming).
• The More-part of the post (might also need renaming).
• The date and time published (might be never or in the feature).
• The date and time created.
• The state of the post (published, draft, etc.).
• The author of the post (both a string-representation and an id is needed. The string-representation might be overridden).