[VSA] Implement the new DB queries

What

With the new database structure in place, we need to implement the query backend. We can implement a switch in lib/gitlab/analytics/cycle_analytics/base_query_builder.rb so we can optionally use the new database and queries.

How

Implement finder classes for the new database tables created in #335386 (closed)

We need two separate finder classes for the following tables:

• analytics_cycle_analytics_merge_request_stage_events
• analytics_cycle_analytics_issue_stage_events

We can start with a base class because the two finders might diverge a bit later.

The following query params we need to support (ideally avoiding joins):

• stage_hash_id
  • This parameter is always required
• root_group_id, group_id, project_id: one of them is always required
• author_username
  • Find the author(s) from the DB and take the id(s). Query the author_id column.
• milestone_title
  • Find the milestone(s) from the DB and take the id(s). Query the milestone_id column.
• start_event_timestamp (greater than)
  • Query the start_event_timestamp column
• end_event_timestamp (less than)
  • Query the end_event_timestamp column