Metadata entities filter API: add facets

**Background**
It would be useful if Cloud Pipeline **metadata entities** filter (`metadataEntity/filter`) endpoint returns per-column statistics, i.e. how many records for each column value.

**Request payload**
Let's introduce *optional* field `facets` (array of column/field names) - if request contains this field, the per-field per-value statistics should be calculated:

- For each metadata entity field (column), a list of unique values should be extracted.
- For each unique field value, total count of entities should be calculated.
- Additionally, the total count of empty/not set values should be calculated.

**Response format**

If facets were requested, response body should contain the following object:

```json
{
"payload": {
"elements": [...],
"totalCount": ...,
"facets": {
"field1": {"counts": {"Unique value 1": 123, "Unique value 2": 3}, "empty": 0},
"field2": {"counts": {}, "empty": 126},
...
}
}
}
```