Add get_most_recent_search_entries_in_history_metadata method to fetch search entries

[cyndichin]

Pull Request checklist

This patch adds a new method get_most_recent_search_entries_in_history_metadata that fetches the most recent entries in the moz_places_metadata table that does not have a null search term. This is to support a new feature in which we want to fetch only the recent search entries instead of fetching the entire history metadata entries.

If we ever decided to add entries with null search terms, then using the convenience method get_most_recent_history_metadata will not suffice and fetches the top 5 entries instead of the top 5 with search terms.

Overall, the new feature we want to support is to show the users recent search terms stored. This PR was also created to support the same feature.

cc: @issammani

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[cyndichin]

Assuming this may conflict with #7101, so need to wait for that PR to merge.

[cyndichin]

Resolved here: 60f5b82

[cyndichin]

@mhammond this PR is now ready to review! Thank you!