Add semantic attribute and pass config during build

• Add semantic attribute similar to suggester (not passed as an Azure attribute but aggregated into the appropriate config).
• Set appropriate limits based on Azure docs (only one title field, only fields that are searchable and retrievable).
• Change the way "_none" is handled so that config unsets (as is, for analyzer selecting _none and saving results in the old config value still being present).

Example index json with this change:

...
"semantic": {
        "defaultConfiguration": null,
        "configurations": [
            {
                "name": "default",
                "prioritizedFields": {
                    "titleField": {
                        "fieldName": "title"
                    },
                    "prioritizedContentFields": [
                        {
                            "fieldName": "field_description"
                        }
                    ],
                    "prioritizedKeywordsFields": [
                        {
                            "fieldName": "field_content_key"
                        },
                        {
                            "fieldName": "name"
                        }
                    ]
                }
            }
        ]
    },
...