Add merge requests data source(s)

Background

To support the MR analytics migration, we will need to add data sources for:

• MR counts
• MR data (to be displayed in the table)

We can re-use the existing MR analytics graphql queries. This is also a good opportunity to simplify the throughput chart query builder.

TODO

• Move/re-use the MR analytics queries as an analytics dashboard data source
• Refactor the throughput chart query builder, the looping logic can be handled in the data source with a simpler query left over