Implement access level methods in UserApiClient.

[DiscoPYF]

fix #282

Implemented the following methods:

• PutDatabaseAccessLevelAsync
• GetDatabaseAccessLevelAsync
• DeleteDatabaseAccessLevelAsync
• GetAccessibleDatabasesAsync
• PutCollectionAccessLevelAsync
• GetCollectionAccessLevelAsync
• DeleteCollectionAccessLevelAsync

Also added related data models and unit tests.

I would like to draw attention on the following endpoint: GET /_api/user/{user}/database/, represented by GetAccessibleDatabasesAsync and the response GetAccessibleDatabasesResponse.

https://www.arangodb.com/docs/stable/http/user-management.html#list-the-accessible-databases-for-a-user

The type of the values in the result property of the response will change if the query parameter full is provided.

Example without full:

"result" : { 
    "_system" : "rw" 
  } 

Example with full:

"result" : { 
    "_system" : { 
      "permission" : "rw", 
      "collections" : { 
        "_fishbowl" : "undefined", 
        "*" : "undefined" 
      } 
   }
}

So I used Dictionary<string, object> for the property type in the C# response model, with a doc comment to explain what to expect.