[DP-3106] Add a way to fetch lists of domains and other search facets

[MatMoore]

Use the aggregateAcrossEntities graphQL query to retrieve lists of domains, tags and other search facets for a result set:

The response from aggregateAcrossEntities looks like this:

{
  "data": {
    "aggregateAcrossEntities": {
      "facets": [
        {
          "field": "domains",
          "displayName": "Domain",
          "aggregations": [
            {
              "value": "urn:li:domain:094dc54b-0ebc-40a6-a4cf-e1b75e8b8089",
              "count": 7,
              "entity": {
                "properties": {
                  "name": "Pet Adoptions"
                }
              }
            },
            {
              "value": "urn:li:domain:7186eeff-a860-4b0a-989f-69473a0c9c67",
              "count": 4,
              "entity": {
                "properties": {
                  "name": "E-Commerce"
                }
              }
            },
            {
              "value": "urn:li:domain:7d64d0fa-66c3-445c-83db-3a324723daf8",
              "count": 3,
              "entity": {
                "properties": {
                  "name": "Marketing"
                }
              }
            }
          ]
        }
      ]
    }
  },
  "extensions": {}
}

I've wrapped the response in a new FacetsResponse object.

facets = client.search_facets()
for option in facets.options('domains'):
  print(option.label)
  print(option.value)
  print(option.count)

This also includes methods for looking up an option by label.

By default, the search_facets returns all the possible options. To render a filtered set of options, pass in the query, filter, and result_types arguments, to match the search method:

facets = client.search_facets(query: "...", filters=[MultiSelectFilter(...)}, result_types=[...])