Refactor contribution analytics data collector

What does this MR do and why?

Splits Gitlab::ContributionAnalytics::DataCollector logic into two additional classes:

• Gitlab::ContributionAnalytics::DataFormatter which receive an array of events and formats data into counts by event.
• Gitlab::ContributionAnalytics::PostgresqlDataCollector execute query to get data from database.

This split is necessary because we will eventually gather data from ClickHouse database the refactoring would easily allow us to introduce Gitlab::ContributionAnalytics::ClickHouseDataCollector

This does not change any contribution analytics behavior.

related to #414604 (closed)

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