feat(search-by-field): Add match highlights to BaseSearchDAO

[dexter-mh-lee]

Add the ability to highlight which part of the search document matched the query. This allows us to highlight any matches that do not show up in the preview (for instance, search for field names in a dataset will highlight the field name that matched the query)

In order for us to control the order of highlighted matches, the PDL matchMetadata field in SearchResultsMetadata holds a list of matched fields for each search element.

*** Tests ***
Once GMS is deployed with this version of gma,
http://localhost:8080/datasets?q=search&input=covid19

{
    "metadata": {
        "urns": [ ... ],
        "matches": [
            {
                "matchedFields": [
                    {
                        "fieldValue": "bigquery-public-data.covid19_usafacts.deaths",
                        "fieldName": "name"
                    }
                ]
            },
            {
                "matchedFields": [
                    {
                        "fieldValue": "bigquery-public-data.covid19_usafacts.confirmed_cases",
                        "fieldName": "name"
                    }
                ]
            },
            {
                "matchedFields": [
                    {
                        "fieldValue": "bigquery-public-data.covid19_italy.data_by_region",
                        "fieldName": "name"
                    }
                ]
            },
            {
                "matchedFields": [
                    {
                        "fieldValue": "bigquery-public-data.covid19_public_forecasts.state_14d",
                        "fieldName": "name"
                    }
                ]
            },
            {
                "matchedFields": [
                    {
                        "fieldValue": "bigquery-public-data.covid19_ecdc.covid_19_geographic_distribution_worldwide",
                        "fieldName": "name"
                    }
                ]
            },
            {
                "matchedFields": [
                    {
                        "fieldValue": "bigquery-public-data.covid19_public_forecasts.japan_prefecture_28d",
                        "fieldName": "name"
                    }
                ]
            },
            {
                "matchedFields": [
                    {
                        "fieldValue": "bigquery-public-data.covid19_public_forecasts.state_14d_historical",
                        "fieldName": "name"
                    }
                ]
            },
            {
                "matchedFields": [
                    {
                        "fieldValue": "bigquery-public-data.covid19_symptom_search.symptom_search_country_weekly",
                        "fieldName": "name"
                    }
                ]
            },
            {
                "matchedFields": [
                    {
                        "fieldValue": "bigquery-public-data.covid19_weathersource_com.county_day_forecast",
                        "fieldName": "name"
                    }
                ]
            },
            {
                "matchedFields": [
                    {
                        "fieldValue": "bigquery-public-data.covid19_weathersource_com.postal_code_day_forecast",
                        "fieldName": "name"
                    }
                ]
            }
        ],
        "searchResultMetadatas": [ ... ]
    },
    "elements": [ ... ],
    "paging": { ... }
}

Checklist

• The PR conforms to DataHub's Contributing Guideline (particularly Commit Message Format)
• Links to related issues (if applicable)
• Tests for the changes have been added/updated (if applicable)
• Docs related to the changes have been added/updated (if applicable)

[shirshanka]

LGTM!

[shirshanka]

Merging this in since it should be backwards compatible.