Menas API: replace arrays with objects as top-level JSON reponses

## Background
As outlined in https://github.com/AbsaOSS/enceladus/pull/1597#discussion_r540049514, Menas API contains top-level arrays in response payloads in a number of endpoints. For reasons outline in the lined discussion, these should be replaced by wrapper objects.

API calls in question:
- `/api/properties/datasets`
- `/api/runs/{summaries, {datasetName}, grouped, /grouped/{datasetName}, /{datasetName}/{datasetVersion}}`
- `/{list, searchSuggestions, /allVersions/{name}}`

## Feature
There should be an (extensible) object as a response in JSON.

## Example
`GET /api/properties/datasets` now yields:
```json
[
{
"name": "datasetProperty1",
...
},
{
"name": "datasetProperty2",
...
}
]
```
The intended response should be e.g.:
```json
{
"propertyDefinitions" : [
{
"name": "datasetProperty1",
...
},
{
"name": "datasetProperty2",
...
}
]
}
```