[BE] Add per-period deploy frequency to Value Stream Analytics

Summary

This issue represents the backend implementation of the issue to Add per-period deploy frequency to Value Stream Analytics (#210079).

Proposal

The Value Stream Analytics backend should return a "deployment frequency" metric:

• "This figure should be calculated as average deploys per day, and adjusted appropriately based on the timeframe selected in the UI" (source).
• It should be accompanied by a unit of "per day".

Some more fine points:

1. If the frequency is a decimal number, then we will display upto one decimal point

2. if hypothetically, a user selected a range of 1 day, we will return the total deployments of that day.