feat(search): Add searchable annotation to maps

[dexter-mh-lee]

We want to index the key/values in the custom properties, but if we index it as an object type, es will try to index each nested field separately and may increase the size of the index if any sources ingests a huge properties bag. Instead, we index it as a list with "key=value" as element. This allows us to search for specific fields with the "customProperties:key=value" query.

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• The PR conforms to DataHub's Contributing Guideline (particularly Commit Message Format)
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• Tests for the changes have been added/updated (if applicable)
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[shirshanka]

LGTM!