implement /api/posts

[bensteUEM]

implement

• /posts for use with Retrieve "ChurchTools" Beiträge and post as recommendation in Communi ChurchWebHelper#36

[bensteUEM]

CT Bugs reported

• issues with /posts/externalposts sample for include - reported case - no mail response -> implement more actions for posts #129
• param "last_post_indentifier" not applied - case 137701 -> followup in GET /api/posts - last_post_indentifier param #131
• group_visibility default param should be "intern" instead of "internal" in documentation of API -> reported case 137702 -> followup in GET /api/posts group_visibility #132
• include is missing [] in swagger documentation - 137723 -> CT documentation update only- no relevance for package